Humans can be carriers of infectious disease not only when ill, but also when seemingly healthy.

Asymptomatic carriers retain pathogens but do not manifest the disease syndrome.  This could be the result of prior illness or a healthy immune system that fails to eliminate small numbers of the pathogen that have become "hidden," such as in S. pyogenes infections behind the tonsilar tissue, HIV and Herpes infections that have become latent, Salmonella residing in the gall bladder or cystic duct, or due to the presence of microbes that cause illness in one host but not another, such as Trichimonas vaginalis infections that are generally asymptomatic in males but result in vaginitis in females.

Subclinical carriers may have an active infection that continues in a reduced form in a part of the body other than the target tissue normally associated with the illness.  One example of this is the persistant infection of tissues associated with the nervous system by the measles virus, resulting in subacute schlerosing panencephalitis.