Protozoa are single-celled organisms which form the basis of food chain. They live in most habitats such as oceans, soil and decaying matter. Some are pathogenic and usually result in waterborne outbreaks. They do not multiply in food but their cysts may remain infectious in foods for a long time. Furthermore, they have a low infective dose, i.e. only small numbers of cysts are required to cause illness. Two pathogenic protozoa causing illness in the UK are Cryptosporidium parvum and Giardia lamblia.
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