Fungi are a diverse group of microbes (bugs). The vast majority do not cause disease in humans, only about 180 of the 250,000 species can cause infection in us. Most of these are moulds and some are yeasts. Moulds are a network of filaments (a bit like threads) formed of many cells. Moulds produce spores which help protect them against unfavourable conditions. These spores can be transmitted in the air or by carriers e.g. animals. Yeasts are small round cells that reproduce by budding. This is when the new cell breaks away from the original parent cell. See 'The Complete Idiot's Guide to Dangerous Diseases & Epidemics' for more information. Some fungi produce chemicals that harm other microbes. Penicillin was originally produced by a fungus.