means The ability to resist or avoid something — whether a disease your body has learned to fight off, or a legal exemption that puts you beyond reach of a rule, tax, or prosecution.

from From Latin 'immunitas,' meaning exemption from public service or duty — built from 'in-' (not) plus 'munus' (a duty, office, or obligation to the community). The same 'munus' gives us 'municipal' and 'commune.' So the original immune person wasn't dodging germs but dodging civic chores; the medical sense — the body 'exempt' from a disease — is a much later borrowing of that legal idea of being let off the hook.