epidemiology / hikmet qubeilat natural history and spectrum of disease natural history of disease...

EPIDEMIOLOGY / HIKMET QUBEILAT

Natural History and Spectrum Natural History and Spectrum of Diseaseof Disease

Natural history Natural history of disease refers to of disease refers to the progress of a disease process in an the progress of a disease process in an individual over time, in the absence of individual over time, in the absence of intervention.intervention.

The process begins with exposure to or The process begins with exposure to or accumulation of factors capable of accumulation of factors capable of causing disease. causing disease.

Without medical intervention, the process Without medical intervention, the process ends with recovery ,disability, or deathends with recovery ,disability, or death



InfectivityInfectivity refers to the proportion refers to the proportion of exposed persons who become of exposed persons who become infected.infected.

PathogenicityPathogenicity refers to the refers to the proportion of infected persons who proportion of infected persons who develop clinical disease.develop clinical disease.

VirulenceVirulence refers to the proportion refers to the proportion of persons with clinical disease who of persons with clinical disease who become severely ill or diebecome severely ill or die



Hepatitis A virus in children has low Pathogenicity Hepatitis A virus in children has low Pathogenicity and low virulence, since many infected children and low virulence, since many infected children remain asymptomatic and few develop severe illness. remain asymptomatic and few develop severe illness.

In persons with good nutrition and health, measles In persons with good nutrition and health, measles virus has high Pathogenicity but low virulence, since virus has high Pathogenicity but low virulence, since almost all infected persons develop the characteristic almost all infected persons develop the characteristic rash illness but few develop the life-threatening rash illness but few develop the life-threatening presentations of measles, pneumonia, or encephalitis.presentations of measles, pneumonia, or encephalitis.

In persons with poorer nutrition and health, measles In persons with poorer nutrition and health, measles is a more virulent disease, with mortality as high as is a more virulent disease, with mortality as high as 5-10%. Finally, rabies virus is both highly pathogenic 5-10%. Finally, rabies virus is both highly pathogenic and virulent, since virtually 100% of all infected and virulent, since virtually 100% of all infected persons (who do not receive treatment) progress to persons (who do not receive treatment) progress to clinical disease and death.clinical disease and death.



The natural history and spectrum of The natural history and spectrum of disease presents challenges to the disease presents challenges to the clinician and to the public health clinician and to the public health worker. Because of the clinical worker. Because of the clinical spectrum, cases of illness diagnosed by spectrum, cases of illness diagnosed by clinicians in the community often clinicians in the community often represent only the “tip of the iceberg.” represent only the “tip of the iceberg.” Many additional cases may be too early Many additional cases may be too early to diagnose or may remain to diagnose or may remain asymptomatic.asymptomatic.



For the public health worker, the For the public health worker, the challenge is that persons with challenge is that persons with inapparent or undiagnosed inapparent or undiagnosed infections may nonetheless be able infections may nonetheless be able to transmit them to others. Such to transmit them to others. Such persons who are infectious but have persons who are infectious but have subclinical disease are called subclinical disease are called carrierscarriers..



Frequently, Frequently, carrierscarriers are persons with are persons with incubating disease or inapparent incubating disease or inapparent infection. Persons with measles, hepatitis infection. Persons with measles, hepatitis A, and several other diseases become A, and several other diseases become infectious a few days before the onset of infectious a few days before the onset of symptoms. On the other hand, carriers symptoms. On the other hand, carriers may also be persons who appear to have may also be persons who appear to have recovered from their clinical illness, such recovered from their clinical illness, such as chronic carriers of hepatitis B virusas chronic carriers of hepatitis B virus


EPIDEMIOLOGYEPIDEMIOLOGY

CHAIN OF INFECTIONCHAIN OF INFECTION


CHAIN OF INFECTIONCHAIN OF INFECTION the traditional model (the traditional model (epi triadepi triad) illustrates ) illustrates

that infectious diseases result from the that infectious diseases result from the interaction of agent, host, and interaction of agent, host, and environment. More specifically, environment. More specifically, transmission occurs when the transmission occurs when the agent agent leaves its leaves its reservoir reservoir or host through a or host through a portal of exitportal of exit, and is conveyed by some , and is conveyed by some mode of transmissionmode of transmission, and enters , and enters through an appropriate through an appropriate portal of entryportal of entry to to infect a susceptible infect a susceptible hosthost. This is . This is sometimes called the chain of infection and sometimes called the chain of infection and is illustrated in Figure 1.18.is illustrated in Figure 1.18.



Reservoir: Reservoir: is the habitat in which an is the habitat in which an infectious agent normally lives, grows, and infectious agent normally lives, grows, and multiplies. Reservoirs include multiplies. Reservoirs include humans, humans, animals, and the environmentanimals, and the environment. The . The reservoir may or may not be the source reservoir may or may not be the source from which an agent is transferred to a from which an agent is transferred to a host. For example, the reservoir of host. For example, the reservoir of Clostridium botulinumClostridium botulinum is soil, but the is soil, but the source of most botulism infections is source of most botulism infections is improperly canned food containing C. improperly canned food containing C. botulinum spores.botulinum spores.


CHAIN OF INFECTION Human reservoirsHuman reservoirs. Many of the common . Many of the common

infectious diseases have human reservoirs. infectious diseases have human reservoirs. Diseases which are transmitted from person to Diseases which are transmitted from person to person without intermediaries include the person without intermediaries include the sexually transmitted diseases, measles, mumps, sexually transmitted diseases, measles, mumps, streptococcal infection, most respiratory streptococcal infection, most respiratory pathogens, and many otherspathogens, and many others. . SmallpoxSmallpox was was eradicated after the last human case was eradicated after the last human case was identified and isolated because humans were the identified and isolated because humans were the only reservoir for the smallpox virus. only reservoir for the smallpox virus. Two types Two types of human reservoir existof human reservoir exist:: persons with symptomatic illnesspersons with symptomatic illness carrierscarriers


CHAIN OF INFECTION

A carrier is a person without apparent disease who is nonetheless capable of transmitting the agent to others. Carriers may be asymptomatic carriers, who never show symptoms during the time they are infected, or may be incubatory or convalescent carriers, who are capable of transmission before or after they are clinically ill. A chronic carrier is one who continues to harbor an agent (such as hepatitis B virus or Salmonella typhi—the agent of typhoid fever) for a extended time (months or years) following the initial infection.


CHAIN OF INFECTION

Carriers commonly transmit disease because they do not recognize they are infected and consequently take no special precautions to prevent transmission. Symptomatic persons, on the other hand, are usually less likely to transmit infection widely because their symptoms increase their likelihood of being diagnosed and treated, thereby reducing their opportunity for contact with others.


CHAIN OF INFECTION Animal reservoirsAnimal reservoirs. Infectious . Infectious

diseases that are transmissible under diseases that are transmissible under normal conditions from animals to normal conditions from animals to humans are called humans are called zoonoseszoonoses .In .In general, these diseases are general, these diseases are transmitted from animal to animal, transmitted from animal to animal, with humans as incidental hosts. with humans as incidental hosts. Such diseases include Such diseases include brucellosisbrucellosis (cows and pigs), (cows and pigs), anthraxanthrax (sheep), (sheep), plague plague (rodents), (rodents), trichinosistrichinosis (swine), (swine), and rabiesand rabies (bats, raccoons, dogs, and (bats, raccoons, dogs, and other mammals).other mammals).


CHAIN OF INFECTION Another group of diseases with Another group of diseases with animal animal

reservoirs are those caused by virusesreservoirs are those caused by viruses transmitted by insects and caused by transmitted by insects and caused by parasites that have complex life cycles, parasites that have complex life cycles, with different reservoirs at different with different reservoirs at different stages of development. Such diseases stages of development. Such diseases include include St. Louis encephalitis and St. Louis encephalitis and malariamalaria (both requiring (both requiring mosquitosmosquitos) and ) and schistosomiasisschistosomiasis (requiring (requiring fresh water fresh water snailssnails). ). Lyme diseaseLyme disease is azoonotic is azoonotic disease of deer incidentally transmitted disease of deer incidentally transmitted to humans by the to humans by the deer tick.deer tick.


CHAIN OF INFECTION

Environmental reservoirsEnvironmental reservoirs. Plants, soil, . Plants, soil, and water in the environment are also and water in the environment are also reservoirs for some infectious agents. reservoirs for some infectious agents. Many fungal agents, such as those Many fungal agents, such as those causing causing histoplasmosishistoplasmosis, live and multiply , live and multiply in the soil. The primary reservoir of in the soil. The primary reservoir of Legionnaires’ bacillusLegionnaires’ bacillus appears to be appears to be pools of water, including those produced pools of water, including those produced by cooling towers and evaporative by cooling towers and evaporative condensers.condensers.


CHAIN OF INFECTION Portal of exitPortal of exit is the path by which an agent is the path by which an agent

leaves the source host. The portal of exit usually leaves the source host. The portal of exit usually corresponds to the site at which the agent is corresponds to the site at which the agent is localized. Thus, localized. Thus, tubercle bacilli and influenza tubercle bacilli and influenza viruses viruses exit the respiratory tract, exit the respiratory tract, schistosomes schistosomes through urinethrough urine, cholera vibrios , cholera vibrios in fecesin feces, , Sarcoptes scabiei Sarcoptes scabiei in scabies skin lesionsin scabies skin lesions, and , and enterovirus 70, an agent of hemorrhagic enterovirus 70, an agent of hemorrhagic conjunctivitis, conjunctivitis, in conjunctival secretions. in conjunctival secretions. Some Some blood borne agents can blood borne agents can exit by crossing the exit by crossing the placenta placenta (rubella, syphilis, toxoplasmosis), (rubella, syphilis, toxoplasmosis), while while others exit by way of the skin (percutaneously) others exit by way of the skin (percutaneously) through cuts or needlesthrough cuts or needles (hepatitis B) (hepatitis B) or blood-or blood-sucking arthropodssucking arthropods (malaria). (malaria).


CHAIN OF INFECTION

Modes of transmission:Modes of transmission: After an agent exits its After an agent exits its natural reservoir, it may be transmitted to a natural reservoir, it may be transmitted to a susceptible host in numerous ways. These modes susceptible host in numerous ways. These modes of transmission are classified as:of transmission are classified as:

DirectDirect Direct contactDirect contact Droplet spreadDroplet spread

IndirectIndirect AirborneAirborne VehicleborneVehicleborne VectorborneVectorborne

MechanicalMechanical BiologicBiologic


CHAIN OF INFECTION

In direct transmissionIn direct transmission, there is , there is essentially immediate transfer of the essentially immediate transfer of the agent from a reservoir to a susceptible agent from a reservoir to a susceptible host by host by direct contact or droplet direct contact or droplet spreadspread. . Direct contactDirect contact occurs through occurs through kissing, skin-to-skin contact, and sexual kissing, skin-to-skin contact, and sexual intercourse. intercourse. Direct contactDirect contact refers also refers also to contact with soil or vegetation to contact with soil or vegetation harboring infectious organisms. harboring infectious organisms.


CHAIN OF INFECTION Infectious mononucleosis (“kissing Infectious mononucleosis (“kissing

disease”)disease”) and and gonorrheagonorrhea are spread from are spread from person-to-person by direct contact. person-to-person by direct contact. HookwormHookworm is spread by direct contact with is spread by direct contact with contaminated soil. contaminated soil. Droplet spreadDroplet spread refers to refers to spray with relatively large, short-range spray with relatively large, short-range aerosols produced by sneezing, coughing, aerosols produced by sneezing, coughing, or even talking. or even talking. Droplet spreadDroplet spread is is classified as direct because transmission is classified as direct because transmission is by direct spray over a few feet, before the by direct spray over a few feet, before the droplets fall to the ground.droplets fall to the ground.


CHAIN OF INFECTION In indirect transmissionIn indirect transmission, an agent is , an agent is

carried from a reservoir to a carried from a reservoir to a susceptible host by susceptible host by suspended air suspended air particles or by animate (vector) or particles or by animate (vector) or inanimate (vehicle) intermediariesinanimate (vehicle) intermediaries. . Most vectors are Most vectors are arthropodsarthropods such as such as mosquitoes, fleas, and ticksmosquitoes, fleas, and ticks. These . These may carry the agent through purely may carry the agent through purely mechanical meansmechanical means. . For example, flies For example, flies carry Shigella on appendages; fleas carry Shigella on appendages; fleas carry Yersinia pestis (agent that carry Yersinia pestis (agent that causes plague) in the gut and deposit causes plague) in the gut and deposit the agent on the skin of a new hostthe agent on the skin of a new host


CHAIN OF INFECTION In mechanical transmissionIn mechanical transmission, the agent , the agent

does not multiply or undergo does not multiply or undergo physiologic changes in the vector. physiologic changes in the vector. This is in contrast to instances in This is in contrast to instances in which an agent undergoes part of its which an agent undergoes part of its life cycle inside a vector before being life cycle inside a vector before being transmitted to a new host. transmitted to a new host. When the When the agent undergoes changes within the agent undergoes changes within the vector, the vector is serving as both an vector, the vector is serving as both an intermediate host and a mode of intermediate host and a mode of transmissiontransmission. This type of indirect . This type of indirect transmission is a transmission is a biologic biologic transmissiontransmission


CHAIN OF INFECTION Guinea worm diseaseGuinea worm disease and and many other many other

vectorborne diseasesvectorborne diseases have complex life cycles have complex life cycles which require an intermediate host. Follow the which require an intermediate host. Follow the life cycle of life cycle of Dracunculus medinensis (Guinea Dracunculus medinensis (Guinea worm)worm) illustrated in illustrated in FigureFigure 1.19 1.19 on page 48. on page 48. What type of transmission does this illustrate?What type of transmission does this illustrate?

Since the agent undergoes part of its life cycle in Since the agent undergoes part of its life cycle in the intermediate host, the agent cannot be the intermediate host, the agent cannot be transmitted by the intermediate host until the transmitted by the intermediate host until the agent has completed that part of its life cycle. agent has completed that part of its life cycle. Therefore, this is an indirect, vectorborne, Therefore, this is an indirect, vectorborne, biologic transmission.biologic transmission.


CHAIN OF INFECTION

Vehicles that may indirectly transmit an agent include food, water, biologic products (blood), and fomites (inanimate objects such as handkerchiefs, bedding, or surgical scalpels). As with vectors, vehicles may passively carry an agent—as food or water may carry hepatitis A virus—or may provide an environment in which the agent grows, multiplies, or produces toxin—as improperly canned foods may provide an environment in which C. botulinum produces toxin.


CHAIN OF INFECTION

Airborne transmission is by particles that are suspended in air. There are two types of these particles: dust and droplet nuclei. Airborne dust includes infectious particles blown from the soil by the wind as well as material that has settled on surfaces and become resuspended by air currents.


CHAIN OF INFECTION Droplet nuclei are the residue of dried

droplets. The nuclei are less than 5 μ (microns) in size and may remain suspended in the air for long periods, may be blown over great distances, and are easily inhaled into the lungs and exhaled. This makes them an important means of transmission for some diseases. Tuberculosis, for example, is believed to be transmitted more often indirectly, through droplet nuclei, than directly, through droplet spread. Legionnaires’ disease and histoplasmosis are also spread through airborne transmission.


CHAIN OF INFECTION

Portal of entryPortal of entry An agent enters a An agent enters a susceptible host through a portal of entry. susceptible host through a portal of entry. The portal of entryThe portal of entry must provide access to must provide access to tissues in which the agent can multiply or tissues in which the agent can multiply or a toxin can act. Often, a toxin can act. Often, organisms use the organisms use the same portal to enter a new host that they same portal to enter a new host that they use to exit the source hostuse to exit the source host. For example, . For example, influenza virusinfluenza virus must exit the respiratory must exit the respiratory tract of the source host and enter the tract of the source host and enter the respiratory tract of the new host.respiratory tract of the new host.


CHAIN OF INFECTION

The route of transmission of many enteric The route of transmission of many enteric (intestinal) pathogenic agents is described (intestinal) pathogenic agents is described as as “fecaloral“fecaloral” because the organisms are ” because the organisms are shed in feces, carried on inadequately shed in feces, carried on inadequately washed hands, and then transferred washed hands, and then transferred through through a vehiclea vehicle ( (such as food, water, or such as food, water, or cooking utensilcooking utensil) to the mouth of a new host. ) to the mouth of a new host. Other portals of entry include the Other portals of entry include the skinskin (hookworm), (hookworm), mucous membranesmucous membranes (syphilis, (syphilis, trachoma), trachoma), and bloodand blood (hepatitis B). (hepatitis B).


CHAIN OF INFECTION

The final link in the chain of infection is a The final link in the chain of infection is a susceptible hostsusceptible host. Susceptibility of a host . Susceptibility of a host depends on depends on genetic factors, specified acquired genetic factors, specified acquired immunity, and other general factorsimmunity, and other general factors which alter which alter an individual’s ability to resist infection or to an individual’s ability to resist infection or to limit pathogenicity. limit pathogenicity. An individual’s genetic An individual’s genetic makeup may either increase or decrease makeup may either increase or decrease susceptibilitysusceptibility. . General factorsGeneral factors which defend which defend against infection include against infection include the skin, mucous the skin, mucous membranes, gastric acidity, cilia in the membranes, gastric acidity, cilia in the respiratory tract, the cough reflex, and respiratory tract, the cough reflex, and nonspecific immune response.nonspecific immune response.


CHAIN OF INFECTION General factorsGeneral factors that may increase susceptibilitythat may increase susceptibility

are are malnutrition, alcoholism, and disease or malnutrition, alcoholism, and disease or therapy which impairs the nonspecific immune therapy which impairs the nonspecific immune responseresponse. . Specific acquired immunitySpecific acquired immunity refers to refers to protective antibodies that are directed against protective antibodies that are directed against a specific agent. Individuals gain protective a specific agent. Individuals gain protective antibodies in two ways: 1) They develop antibodies in two ways: 1) They develop antibodies in response to infection, vaccine, or antibodies in response to infection, vaccine, or toxoid; immunity developed in these ways is toxoid; immunity developed in these ways is called called active immunityactive immunity. 2) They acquire their . 2) They acquire their mothers’ antibodies before birth through the mothers’ antibodies before birth through the placenta or they receive injections of antitoxins placenta or they receive injections of antitoxins or immune globulin; immunity that is acquired or immune globulin; immunity that is acquired in these ways is called in these ways is called passive immunitypassive immunity..


CHAIN OF INFECTION

Note that the chain of infection may be Note that the chain of infection may be interrupted when an agent does not find interrupted when an agent does not find a susceptible host. This may occur if a a susceptible host. This may occur if a high proportion of individuals in a high proportion of individuals in a population is resistant to an agent. These population is resistant to an agent. These persons limit spread to the relatively few persons limit spread to the relatively few who are susceptible by reducing the who are susceptible by reducing the probability of contact between infected probability of contact between infected and susceptible persons. and susceptible persons. This concept is This concept is called herd immunity.called herd immunity.


CHAIN OF INFECTION The degree of The degree of herd immunityherd immunity

necessary to prevent or abort an necessary to prevent or abort an outbreak varies by disease. In theory, outbreak varies by disease. In theory, herd immunityherd immunity means that not means that not everyone in a community needs to be everyone in a community needs to be resistant (immune) to prevent disease resistant (immune) to prevent disease spread and occurrence of an outbreak. spread and occurrence of an outbreak. In practice, In practice, herd immunityherd immunity has not has not prevented outbreaks of measles and prevented outbreaks of measles and rubella in populations with immunity rubella in populations with immunity levels as high as 85 to 90%levels as high as 85 to 90%


CHAIN OF INFECTION

One problem is that, in highly One problem is that, in highly immunized populations, the immunized populations, the relatively few susceptible persons relatively few susceptible persons are often clustered in population are often clustered in population subgroups, usually defined by subgroups, usually defined by socioeconomic or cultural socioeconomic or cultural factors. If the agent is introduced factors. If the agent is introduced into one of these subgroups, an into one of these subgroups, an outbreak may occur.outbreak may occur.


CHAIN OF INFECTION

Implications for public healthImplications for public health By By knowing how an agent exits and enters knowing how an agent exits and enters a host, and what its modes of a host, and what its modes of transmission are, we can determine transmission are, we can determine appropriate control measures. In appropriate control measures. In general, we should direct control general, we should direct control measures against the link in the measures against the link in the infection chain that is most susceptible infection chain that is most susceptible to interference, unless practical issues to interference, unless practical issues dictate otherwise.dictate otherwise.


CHAIN OF INFECTION

For some diseases, For some diseases, the most appropriate the most appropriate intervention may be directed at intervention may be directed at controlling or eliminating the agent at controlling or eliminating the agent at its sourceits source. In the hospital setting, . In the hospital setting, patients may be treated and/or isolated, patients may be treated and/or isolated, with appropriate “enteric precautions,” with appropriate “enteric precautions,” “respiratory precautions,” “universal “respiratory precautions,” “universal precautions,” and the like for different precautions,” and the like for different exit pathways. In the community, soil exit pathways. In the community, soil may be decontaminated or covered to may be decontaminated or covered to prevent escape of the agent.prevent escape of the agent.


CHAIN OF INFECTION

Sometimes, Sometimes, we direct interventions at the we direct interventions at the mode of transmissionmode of transmission. For direct . For direct transmission, we may provide treatment transmission, we may provide treatment to the source host or educate the source to the source host or educate the source host to avoid the specific type of contact host to avoid the specific type of contact associated with transmission. In the associated with transmission. In the hospital setting, since most infections are hospital setting, since most infections are transmitted by direct contact, transmitted by direct contact, handwashing is the single most important handwashing is the single most important way to prevent diseases from spreading.way to prevent diseases from spreading.


CHAIN OF INFECTION

For vehicle borne transmissionFor vehicle borne transmission, we may , we may decontaminate or eliminate the vehicle. decontaminate or eliminate the vehicle. For For fecal-oral transmissionfecal-oral transmission, we may also try to , we may also try to reduce the risk of contamination in the future reduce the risk of contamination in the future by rearranging the environment and educating by rearranging the environment and educating the persons involved in better personal the persons involved in better personal hygiene. hygiene. For airborne transmissionFor airborne transmission, we may , we may modify ventilation or air pressure, and filter or modify ventilation or air pressure, and filter or treat the air. treat the air. For vector borne transmissionFor vector borne transmission, we , we usually attempt to control (i.e., reduce or usually attempt to control (i.e., reduce or eradicate) the vector population.eradicate) the vector population.


CHAIN OF INFECTION

Finally, we may apply measures that Finally, we may apply measures that protect protect portals of entryportals of entry of a susceptible of a susceptible potential host or reduce the susceptibility potential host or reduce the susceptibility of the potential host. For example, of the potential host. For example, a a dentist’s mask and gloves are intended to dentist’s mask and gloves are intended to protect the dentist from a patient’s blood, protect the dentist from a patient’s blood, secretions, and droplets, as well to protect secretions, and droplets, as well to protect the patient from the dentist.the patient from the dentist. Prophylactic Prophylactic antibiotics and vaccination are strategies antibiotics and vaccination are strategies to improve a potential host’s defenses.to improve a potential host’s defenses.


Epidemic Disease Epidemic Disease OccurrenceOccurrence

Level of disease Level of disease The amount of a The amount of a particular disease that is usually present in a particular disease that is usually present in a community is the baseline level of the community is the baseline level of the disease. This level is not necessarily the disease. This level is not necessarily the preferred level, which should in fact be zero; preferred level, which should in fact be zero; rather it is the observed level. Theoretically, rather it is the observed level. Theoretically, if no intervention occurred and if the level is if no intervention occurred and if the level is low enough not to deplete the pool of low enough not to deplete the pool of susceptible persons, the disease occurrence susceptible persons, the disease occurrence should continue at the baseline level should continue at the baseline level indefinitely. indefinitely.



Thus, the baseline level is often Thus, the baseline level is often considered the expected level of the considered the expected level of the disease. For example, over the past disease. For example, over the past 4 years the number of reported 4 years the number of reported cases of poliomyelitis has ranged cases of poliomyelitis has ranged from 5 to 9. Therefore, assuming from 5 to 9. Therefore, assuming there is no change in population, we there is no change in population, we would expect to see approximately 7 would expect to see approximately 7 reported cases next year.reported cases next year.



Different diseases, in different Different diseases, in different communities, show different patterns of communities, show different patterns of expected occurrence: a persistent level expected occurrence: a persistent level of occurrence with a low to moderate of occurrence with a low to moderate disease level is referred to as an disease level is referred to as an endemic levelendemic level a persistently high level of a persistently high level of occurrence is called a occurrence is called a hyperendemic hyperendemic level level an irregular pattern of occurrence, an irregular pattern of occurrence, with occasional cases occurring at with occasional cases occurring at irregular intervals is called sirregular intervals is called sporadicporadic..



Occasionally, the level of disease rises Occasionally, the level of disease rises above the expected level. When the above the expected level. When the occurrence of adisease within an area is occurrence of adisease within an area is clearly in excess of the expected level for a clearly in excess of the expected level for a given time period, it is called given time period, it is called an epidemican epidemic. . Public health officials often use the term Public health officials often use the term outbreakoutbreak, which means the same thing, , which means the same thing, because it is less provocative to the public. because it is less provocative to the public. When an epidemic spreads over several When an epidemic spreads over several countries or continents, affecting a large countries or continents, affecting a large number of people, it is called number of people, it is called a pandemica pandemic..



Epidemics occur when an agent and susceptible Epidemics occur when an agent and susceptible hosts are present in adequate numbers, and the hosts are present in adequate numbers, and the agent can effectively be conveyed from a source agent can effectively be conveyed from a source to the susceptible hosts. More specifically, an to the susceptible hosts. More specifically, an epidemic may result from the following:epidemic may result from the following:

1.1. A recent increase in amount or virulence of the A recent increase in amount or virulence of the agentagent

2.2. The recent introduction of the agent into a setting The recent introduction of the agent into a setting where it has not been beforewhere it has not been before

3.3. An enhanced mode of transmission so that more An enhanced mode of transmission so that more susceptibles are exposedsusceptibles are exposed

4.4. Some change in the susceptibility of the host Some change in the susceptibility of the host response to the agentresponse to the agent

5.5. Factors that increase host exposure or involve Factors that increase host exposure or involve introduction through new portals of entryintroduction through new portals of entry