insusceptible

Insusceptible is an adjective meaning not susceptible; resistant or immune to something, and not easily affected by a disease, influence, or condition. In scientific contexts, it describes an organism or individual that does not become infected or affected by a particular pathogen or toxin. In everyday language, the term is less common than immune or resistant, but it remains a valid descriptor in technical writing.

Etymology and usage history suggest it comes from Latin roots in- (not) and susceptibilis (able to be affected), forming insusceptibilis in older English texts. The word entered English through scientific and legal prose, with greater prevalence in earlier centuries; today it is primarily found in technical or specialized writings.

In practice, insusceptible is most often applied in biology and medicine. It can refer to hosts or varieties that do not contract a disease under exposure to a specific pathogen, due to genetic traits, preexisting immunity, or structural or metabolic defenses. It can also describe a non-responsive reaction to a stimulus or treatment, though this usage is less common. The term should be distinguished from immune, which implies active protection, and from resistant, which denotes a reduction in risk rather than absolute non-susceptibility. The noun form is insusceptibility.

Related terms include susceptibility, immunity, and resistance. Insusceptible is best used when describing a definite lack of susceptibility to a particular agent or condition, rather than a general or absolute state of protection.