Infectious diseases kill more people worldwide than any other single cause. Infectious diseases are caused by germs. Germs are tiny living things that are found everywhere - in air, soil and water. You can get infected by touching, eating, drinking or breathing something that contains a germ. Germs can also spread through animal and insect bites, kissing and sexual contact. Vaccines, proper hand washing and medicines can help prevent infections.
There are four main kinds of germs:
Bacteria - one-celled germs that multiply quickly and may release chemicals which can make you sick
Viruses - capsules that contain genetic material, and use your own cells to multiply
Fungi - primitive vegetables, like mushrooms or mildew
Protozoa - one-celled animals that use other living things for food and a place to live