To be immune to something

Origin

Imagine ancient Rome, where a privileged few were declared immunis, meaning "exempt from public service or burden." These individuals were literally free from civic duties or taxes—a coveted state of being. As history unfolded and humanity grappled with devastating plagues, a new kind of "exemption" emerged: those who survived a disease often seemed protected from it thereafter. This observation, that certain individuals were inexplicably free from recurring illness, drew a powerful parallel to the legal immunis. The word was eventually adopted into medical lexicon, transforming from a civic status to a biological state, signifying an ultimate freedom—the body's triumphant exemption from disease.