The infection rate was the rate at which individuals became infected with a disease.
In 2151, Esaak reported to Doctor Phlox that the rate of infection amongst Valakians was one out of three. (ENT: "Dear Doctor")
In 2364, Starbase 14 transmitted data about the infection rate of Anchilles fever on Styris IV to USS Enterprise-D. (TNG: "Code of Honor")