/ɪnˈfɛkt/
[verb] To pass a disease or harmful microorganism to a person, animal, or plant. It can also mean to influence someone's feelings or attitudes, often in a negative way.
Before bacteria were discovered, 'infect' simply meant to dye or stain something — colour was the original contagion.
The word 'infect' travels to us from the Latin verb 'inficere', which was built from two pieces: the prefix 'in-', meaning 'into', and 'facere', meaning 'to make' or 'to do'. In its earliest Latin life, 'inficere' had nothing to do with illness at all. It meant to dip something into a substance, to stain it, or to dye it a colour. A cloth could be 'infected' with purple dye just as easily as a wound might be infected with poison. The word carried the idea of something being fundamentally altered by contact with another substance — changed from the inside out.
Over time, Latin writers began using 'inficere' in a darker sense. Since poisons and harmful substances also 'entered into' the body and changed it, the word gradually took on the meaning of contaminating or corrupting something. The past participle of 'inficere' was 'infectus', meaning 'stained', 'tainted', or 'spoiled'. From this past participle, the Romans derived the noun 'infectio', meaning a staining or a corruption. It is from 'infectus' and 'infectio' that English would eventually draw its key words.
The word moved into Old French as 'infecter', keeping the sense of poisoning or corrupting, before passing into Middle English during the 14th century. Early English texts used 'infecten' and later 'infect' to describe the spreading of plague, poison, and moral corruption with almost equal ease. The great epidemics of the medieval period — including the Black Death, which swept Europe from 1347 onwards — gave the word urgent, terrible relevance, and it became firmly attached to the idea of disease spreading from one person to another.
What is striking about the history of 'infect' is how long it took for the word to catch up with the science it now describes. For centuries, people used 'infect' to describe the spread of disease without understanding exactly how that spread happened. Theories of 'miasma' — the idea that bad air carried illness — dominated medical thinking well into the 19th century. The word 'infect' was doing real conceptual work long before germ theory, proposed by scientists like Louis Pasteur and Robert Koch in the 1860s and 1870s, gave it a precise biological meaning.
The word also developed a vivid metaphorical life alongside its medical one. Writers have long used 'infect' to describe the spreading of ideas, emotions, and behaviours — laughter can infect a room, enthusiasm can infect a team, and fear can infect an entire population. This figurative use returns, in a poetic way, to the word's very oldest meaning: the idea of one thing entering into another and changing it completely. The Latin root 'facere' is also the ancestor, through various paths, of English words like 'fact' (Latin 'factum' → English 'fact'), 'manufacture' (Latin 'manu facere' → French 'manufacturer' → English 'manufacture'), and 'affect' (Latin 'afficere' → English 'affect'), connecting 'infect' to a vast family of words all built on the simple idea of doing or making something happen.