Most people know that a germ is a very small living organism that causes disease. Germs are microbes, namely that part of the living world that can usually be seen only under a microscope, if at all. The world of the microbe is enormously diverse and includes viruses such as smallpox and HIV, bacteria, fungi and moulds, small worms and little animal-like organisms that cause diseases such as malaria. Most microbes are harmless to us humans, and some are beneficial not only in food such as cheeses or yoghurt, but also by playing an important role in the normal functioning of our gut.