Origins
The word 'infect' entered English in the fourteenth century from the Latin past participle 'infectus,' from the verb 'inficere,' meaning 'to stain, to dye, to taint, to poison.' The verb is a compound of 'in-' (into) and 'facere' (to make, to do), with the literal sense of 'to put into' or 'to dip into.' The path from dyeing to disease is a striking semantic journey: to dip cloth into dye is to stain it permanently, and from staining came the idea of tainting or corrupting, which naturally extended to the corruption of the body by disease.
The Latin verb 'facere' is one of the most productive verbs in the language, descending from the Proto-Indo-European root *dʰeh₁- (to put, to place, to make). Through its various prefixed forms, it has given English an enormous vocabulary: 'affect' (to act upon), 'effect' (to bring about), 'defect' (something undone or lacking), 'perfect' (thoroughly made), 'confect' (to make together, hence confection), 'manufacture' (to make by hand), and dozens more. The '-fect' and '-fic' forms in English all trace back to this single Latin verb.
In Classical Latin, 'inficere' had a range of meanings that reflect its etymological transparency. Pliny the Elder used it for dyeing wool. Cicero employed it metaphorically for the corruption of morals. Medical writers like Celsus used it for the contamination of wounds. This breadth of usage — physical staining, moral corruption, and bodily contamination — passed into the Romance languages and eventually into English.
Middle English
When 'infect' arrived in Middle English, it carried both physical and moral senses. Medieval writers spoke of sin infecting the soul just as readily as plague infecting the body. The two senses reinforced each other in the medieval mind, which saw disease as intimately connected to moral and spiritual corruption. The Black Death (1347–1351), which devastated Europe during exactly the period when 'infect' was entering English, gave the word an urgency and terror that colored its usage for centuries.
The scientific understanding of infection underwent a revolution in the nineteenth century with the germ theory of disease, pioneered by Louis Pasteur and Robert Koch. Before germ theory, 'infection' was understood through the 'miasma' theory — the belief that diseases were caused by bad air or noxious vapors. The word 'infect' was compatible with both theories: whether contamination came from invisible organisms or poisonous air, the word's core sense of 'tainting' or 'corrupting' applied equally well.
The derivative 'disinfect' (to remove infection) appeared in the late sixteenth century, and 'disinfectant' followed in the eighteenth century. Joseph Lister's development of antiseptic surgery in the 1860s made 'disinfection' a central concept in medicine. The modern vocabulary of public health — 'infectious disease,' 'infection control,' 'nosocomial infection' (hospital-acquired) — testifies to the word's enduring medical centrality.
Figurative Development
In computing, 'infect' acquired new life in the late twentieth century with the emergence of computer viruses. The biological metaphor was deliberate: a computer virus 'infects' a system, 'replicates' itself, and 'spreads' to other machines. The word proved so apt that the entire vocabulary of computer security borrows heavily from epidemiology: 'quarantine,' 'worm,' 'vector,' 'payload.'
The metaphorical sense of 'infect' — to influence powerfully, often negatively — remains vigorous. Ideas, emotions, and attitudes are said to 'infect' people, and laughter is famously 'infectious.' This metaphorical usage preserves the word's oldest Latin sense: a stain that spreads, a taint that cannot easily be removed.