means The invasion and multiplication of harmful microorganisms—bacteria, viruses, fungi—inside the body, often producing illness.

from From Latin 'inficere,' literally 'to put in' or 'to dip into,' built from 'in-' (into) plus 'facere' (to do, to make)—the same 'facere' lurking inside 'factory' and 'fact.' Its early sense was about staining or tainting, as if dipping something clean into something foul. The disease meaning grew from that idea of contamination, the notion that corruption could be poured into a body. The germ part came much later; for centuries people knew the word long before they knew what was actually doing the dipping.