Traditionally we consider infection to arise from viruses, bacteria and parasites. Prions are infectious proteins without any nucleic acids, and therefore do not represent living things. Despite this, they have the ability to replicate themselves and cause diseases such as mad cow disease (bovine spongiform encepthalopathy) and human Creutzfeldt--Jakob disease (CJD). This article discusses how prions were first discovered and what has since been elucidated about their effects in various organisms. The mechanisms of protein misfolding in relation to prion biology are also examined.