Infection
Etymology
From Latin "inficere" (to dip in, stain, taint, spoil), from "in-" (into) and "facere" (to make, do). The original image is of dyeing or contaminating something by dipping it — a body "stained" by an invading organism.
noun · /in-FEK-shun/
The invasion and multiplication of harmful microbes inside the body.
What it names
A handle for the moment a pathogen takes hold and grows, as opposed to mere exposure or a non-living cause.
Example
A cut becomes an infection only once bacteria settle in and start to multiply.
Related idea
An infection is by living organisms — bacteria, viruses, fungi — that reproduce inside you, while an infestation is by larger parasites like lice or worms that live on or in the body without merging into its tissue chemistry. Both are invasions; only infection is microbial.