As bacteria and viruses are very small they can travel in the air, in water and in food. They can also be passed on by touch. Bacteria travel through the air as spores. A bacterial spore has a thick wall which encloses the bacterium. The thick wall prevents the bacterium from drying out and also insulates it from heat. The virus does not make a spore. Its coat is thick enough to protect its contents from the drying action of the air.

Bacteria and viruses can enter the body from the air or from food, touch and water. They can land on the skin and enter the body when the skin is cut. When bacteria enter the body they live and feed on moist surfaces such as the lining of the breathing system. If there is enough food they may breed as often as once every twenty minutes. Bacteria may produce poisons as they feed. As the numbers of bacteria increase the amount of poison produced also increases. This poison can cause disease.

Viruses empty their genes into cells. The genes make their way to the cell nucleus and make copies of themselves. Each new set of genes makes a case and leaves the cell. As the genes make their copies they damage the nucleus so much that the cell dies. Each new virus attacks another cell and the copying process is repeated. In time the effects of the virus attack cause disease.

The body is not defenceless against the microbes that invade it. It responds by increasing the number of white cells in the blood. These gather at the areas of infection and surround, then digest the bacteria and viruses.

The body cells that are attacked by microbes produce chemicals which make the body temperature rise. When our temperature is raised we say we have a fever. As our body wins its battle with the microbes the temperature falls back to normal and the fever is over.