immune

From Latin immūnis, "exempt from public service," formed from in- ("not") and mūnus ("service"). This passed through Middle French immun into Middle English before arriving in English.

The word began as a legal and civic term for someone excused from duties or taxes, and only later came to describe the body's exemption from disease.

It shares its root mūnus with words built around the idea of duty or service, though no specific English cognate is given in the record here.