The ability of pathogens to become established within or on the tissues of a host — the capability to invade, colonize, and begin multiplying. A highly infective organism needs only a small dose to cause illness, which is why dose-response matters in food safety: for pathogens like norovirus or E. coli O157, even slight contamination of food can lead to outbreaks. Infectivity varies with the pathogen, the food matrix, and the host’s susceptibility, and quantitative risk assessments use dose-response models built on infectivity data.