Whilst there are many different species of microbes, only a very small proportion cause disease in humans. Most microbes belong to one of four groups: viruses, bacteria, fungi and protozoa.
Viruses: these are among the smallest microbes and are not cells. They are made of molecules of DNA or RNA, which contain the genes of the virus, and are surrounded by a protein coat. Viruses are shaped like rods, spheres or are multisided and some can look like tadpoles. Most viruses invade living cells such as those in the human body, multiply and produce other viruses. Viruses attack specific cells and in humans they attack particular cells in the body such as the liver, respiratory system or blood cells. Once inside the cell the viral nucleic acid enters the nucleus of the host cell where it uses the cell’s own mechanism to make new virus offspring and the cell becomes a factory producing new viruses.
Bacteria: bacteria are made of one cell and under a microscope look like balls, rods or spirals. Different bacteria require different conditions to survive. Some need oxygen (aerobic bacteria) and others do not (anaerobic bacteria), whilst some can survive with or without oxygen. Each bacterium needs food for energy and to build and multiply. Some bacteria produce poisons called toxins that can seriously affect health. One example is botulism a severe form of food poisoning. It affects the nerves and is caused by a toxin from Clostridium botulinum bacteria.
Fungi: these are primitive vegetables that can be found in the air, soil, plants and in water. Those most familiar to us include mushrooms, yeast, mould and mildew. Fungal infections can affect your skin, nails, body hair, organs like the lungs and the nervous system.
Protozoa: these are a group of one-celled animals found living in water, soil and decaying matter. In humans protozoa usually cause diseases such as malaria, which is caused by a protozoan parasite.
Prions: are replicating agents made almost completely of protein. Infection is due to a PrP protein present in an abnormal arrangement, which seems to cause disease by replacing normal proteins on the surface of host cells. The main prion disease is variant CJD (vCJD) and can be found in tissues such as lymph nodes, tonsils and spleen. Prion proteins are highly resistant to conventional methods of decontamination.