From Latin 'inficere' (to stain), from 'facere' (to make) — disease as a spreading stain, originally a dyeing metaphor.
To contaminate with a disease-causing organism; to affect or corrupt with harmful ideas, feelings, or influences.
From Latin 'infectus,' past participle of 'inficere,' meaning 'to stain, dye, taint, poison,' composed of 'in-' (into) and 'facere' (to make, to do). The literal sense was 'to put into, to dip into' (as in dyeing cloth), which evolved into 'to stain' and then 'to taint, to corrupt.' The medical sense of contaminating with disease developed from the broader notion of tainting or corrupting. The PIE root behind 'facere' is *dʰeh₁- (to put, to place, to make). Key
The original Latin sense of 'inficere' was simply 'to dip into dye' — the journey from dyeing cloth to disease transmission reflects an ancient intuition that contamination spreads like a stain, centuries before germ theory confirmed the metaphor.