Regular exercise and a balanced diet are needed to keep the body healthy. Too little food leads to a person being underweight and prone to certain types of illness, while too much food and not enough exercise leads to a person being overweight and other types of ill health.NutrientsA mixture of different types of food in the correct amounts is needed to maintain health.The main food groups arecarbohydrates,fatsandproteins.This table summarises some typical sources of these nutrients and why our bodies need them.The main food groupsFood groupFound inNeeded by our bodies for:

Carbohydrates

potatoes, pasta, bread, bananas, sugar and ricea source of energy for life processes

Fats

cheese, butter, margarine and oilsa source of energy for life processes: fats are also needed to make cell membranes and to insulate our bodies

Proteins

meat, fish, eggs and cheesegrowth and repair - building cells

Mineral ions andvitaminsare also important in a healthy diet. They are needed in small amounts for healthy functioning of the body.Imbalanced dietsAn imbalanced diet causes a person to become malnourished. For example: too little food may lead to a person being underweight too much food may lead to a person being overweight.A poor diet may also lead to deficiency diseases. For example, too little vitamin D in the diet can lead to rickets, which affects the proper growth of the skeleton. Type 2 diabetes is also a problem related to poor diet

if someones diet consists of food with a lower energy content than the amount of energy their body uses, the person will lose body mass.Metabolic rateRespiration is a chemical reaction that allows cells to release energy from food. The metabolic rate is the speed at which such chemical reactions take place in the body and varies because of several factors, including age, gender and inherited factors.Metabolic rate is also affected by the: proportion of muscle to fat in the body amount of exercise and other physical activity.The metabolic rate increases as we exercise and stays high for a while afterwards. People who exercise regularly are usually healthier than people who don't.

A molecular model of a cholesterol moleculeOther factors affect health too, including inherited factors. For example, the cholesterol level in the blood can affect health. Cholesterol is made in the liver and is needed for healthy cell membranes. However, too much cholesterol in the blood increases the risk of heart disease and diseased arteries.Defending agenised infections Pathogens are microorganisms - such as bacteria and viruses - that cause disease. Bacteria release toxins, and viruses damage our cells. White blood cells can ingest and destroy pathogens by producing antibodies that destroy the infectious microorganisms, and antitoxins to neutralise toxins created as a by-product.Pathogens: bacteria Structure of a salmonella bacterium cellBacteria and viruses are the main types of pathogen. Bacteria are microscopic organisms. They come in many shapes and sizes, but even the largest are only 10 micrometres long - that's 10 millionths of a metre.Bacteria are living cells and, in favourable conditions, can multiply rapidly. Once inside the body, they release poisons or toxins that make us feel ill. Diseases caused by bacteria include:

food poisoning cholera typhoid whooping cough.

Pathogens: virusesViruses are many times smaller than bacteria. They are among the smallest organisms known and consist of a fragment of genetic material inside a protective protein coat.

A hepatitis C virus showing DNA enclosed in a protein coat.Viruses can only reproduce inside host cells and they damage the cell when they do this. A virus can get inside a cell and, once there, take over and make hundreds of thousands of copies of itself. Eventually the virus copies fill the whole host cell and burst it open. The viruses are then passed out in the bloodstream, the airways, or by other routes.Diseases caused by viruses include: influenza (flu) colds measles mumps rubella chicken pox AIDs.

White blood cells

A white blood cell ingesting disease-causing bacteria.White blood cells can: ingest pathogens and destroy them produce antibodies to destroy particular pathogens produce antitoxins that counteract the toxins released by pathogens.In a written examination, it is easy to get carried away with metaphors about invaders and battles: stick to the point. Note that: the pathogens are not the disease - theycausethe disease white blood cells do not eat the pathogens - theyingestthem antibodies and antitoxins are not living things - they are specialisedproteins.

ImmunityPathogens contain certain chemicals that are foreign to the body. These chemicals are calledantigens. Certain white blood cells, calledlymphocytes, can produce specific antibodies to kill a particular pathogen.AntibodiesAntibodies are proteins. They can neutralise pathogens in a number of ways. For example, they can: bind to pathogens and damage or destroy them coat pathogens, clumping them together so that they are easily ingested by white blood cells calledphagocytes.Each lymphocyte produces a specific type of antibody - a protein that has a chemical 'fit' to a certain antigen. When a lymphocyte with the appropriate antibody meets the antigen, the lymphocyte reproduces quickly and makes many copies of the antibody to kill the pathogen.

Protecting against pathogensVaccinationVaccination causes the body to produce enough white blood cells to protect itself against a pathogen. Antibiotics are effective against bacteria, but not against viruses. Some strains of bacteria are resistant to antibiotics. So people can be immunised against a pathogen through vaccination. Different vaccines are needed for different pathogens. For example, the MMR vaccine is used to protect children against measles, mumps and rubella (German measles).Vaccination involves putting a small amount of an inactive form of a pathogen, or dead pathogen, into the body.Vaccines can contain: live pathogens treated to make them harmless harmless fragments of the pathogen dead pathogens.These all act as antigens. When injected into the body, they stimulate white blood cells to produce antibodies against the pathogen. If the person does get infected by the pathogen later, their body can respond in the same way as if they had had the disease before. If a large proportion of the population is immune to a particular pathogen, the spread of that pathogen is greatly reduced.

hygieneOne simple way to reduce the risk of infection is to maintain personal hygiene and to keep hospitals clean. In the 19th century, Ignaz Semmelweiss realised the importance of cleanliness in hospitals. Semmelweiss insisted that doctors should wash their hands before examining patients, something that was not common at the time. This policy greatly reduced the number of deaths from infectious diseases in his hospital. Unfortunately, although his ideas were successful, they were ignored at the time because people did not know that diseases were caused by pathogens that could be killed.

MedicinesSome medicines help torelieve the symptomsof a disease while otherskillthe infectious pathogens.Painkillers

Painkillers like paracetamol and aspirin are available 'over-the-counter' at pharmacies.Painkillers helps to relieve the symptoms of an infectious disease, but they do not kill the pathogens involved. For example, paracetamol, aspirin and morphine block nerve impulses from the painful part of the body, or block nerve impulses travelling to the part of the brain responsible for perceiving pain.Antibiotics

A bacterium damaged and distorted by penicillinAntibiotics are substances that kill bacteria or stop their growth. They do not work against viruses because they live and reproduce inside cells. It is difficult to develop drugs that kill viruses without also damaging the bodys tissues.Penicillin, the first antibiotic, was discovered in 1928 by Alexander Fleming. He noticed that some bacteria he had left in a Petri dish had been killed by naturally occurring penicillium mould. Since the discovery of penicillin, many other antibiotics have been discovered and developed.Different antibiotics work in different ways. It is important that specific bacteria should be treated using specific antibiotics. This table gives some examples (you do not need to remember this information).How some common antibiotics workAntibioticHow it works

penicillinbreaks down cell walls

erythromycinstops protein synthesis

ciprofloxacinstopsDNAreplication

The use of antibiotics has prevented many deaths from infectious bacterial diseases.

Antibiotic resistanceBacterial strains can develop resistance to antibiotics. This happens because of natural selection. In a large population of bacteria, there may be some cells that are not affected by the antibiotic. These cells survive and reproduce, producing even more bacteria that are not affected by the antibiotic.MRSAMRSA is the acronym for 'methicillin-resistant Staphylococcus aureus'. It's very dangerous because it's a strain of bacterium that is resistant to most antibiotics. To slow down or stop avoid the unnecessary use of antibiotics complete the full course.The appearance of resistant strains of bacteria means that vaccinations and antibiotics may no longer work. As people are not immune to it, and there is no effective treatment, a resistant strain will spread rapidly. New antibiotics must be developed as a result.

Growing microorganisms in the lab

E. coli bacteria (Photo from Rocky Mountain Laboratories, NIAID, NIH)In 1878 Robert Koch discovered how to grow bacteria in a Petri dish (named after his assistant Julius Petri). He was able to discover which bacteria caused certain diseases, including TB and cholera. Scientists still grow microorganisms in the lab so that they can be investigated.Culturing microorganismsThe action of antibiotics and disinfectants can be investigated using cultures of microorganisms (populations of microorganisms that have been grown for a purpose). It is important that the cultures are uncontaminated by other microorganisms, so sterile conditions are needed: the Petri dishes, nutrient agar jelly and other culture media must be sterilised the inoculating loops used to transfer microorganisms must be sterilised (usually by passing the metal loop through a Bunsen burner flame) the lid of the Petri dish is sealed with sticky tape to stop microorganisms from the air getting in and contaminating the culture.Safety in the labBacteria grow and reproduce more quickly when they are warm than when they are cold. It would be dangerous to incubate (keep and grow) cultures at temperatures close to body temperature (37C) because doing so might allow the growth of pathogens harmful to health. So the maximum temperature used in school and college labs is 25C. However, higher temperatures can be used industrially, and these produce faster growth.