An infected organism that does not show symptoms during illness is referred to as a:

An infected organism that does not show symptoms during illness is referred to as a carrier. Carriers are important in epidemiology because they can transmit pathogens to others without displaying signs of the disease themselves. This asymptomatic state means they can unknowingly facilitate the spread of infection within populations, complicating efforts to control outbreaks.

In contrast, vectors, such as insects, actively transmit pathogens from one host to another but are not necessarily infected themselves. A host refers to an organism that harbors a pathogen, but a host can exhibit symptoms of the disease. A pathogen is the actual microorganism—like bacteria or viruses—that causes disease and is not an infected organism. Thus, the term "carrier" is distinct and specifically denotes the asymptomatic but infectious nature of the organism in question.