'germ theory' originates from English combining the word 'germ' (ultimately from Latin 'germen') and the word 'theory' (from Greek 'theoria'), where 'germen' meant 'sprout' or 'seed' and 'theoria' meant 'contemplation' or 'a looking at'.

'germ' came into English via Latin 'germen' (and Old French 'germe') meaning 'seed' or 'bud'; by the 19th century 'germ' was extended to mean a minute organism causing disease. 'Theory' comes from Greek 'theoria' through Latin and Old French. The compound phrase 'germ theory' developed in the mid-19th century to name the idea that microbes cause disease, and it became the standard term for that scientific concept.

Initially 'germ' meant 'seed' or 'offspring'; over time it came to denote minute living organisms (microbes), so 'germ theory' came to mean the scientific explanation that such microbes cause disease.