A subclinical infection—sometimes called a preinfection or inapparent infection—is an infection by a pathogen that causes few or no signs or symptoms of infection in the host.[1] Subclinical infections can occur in both humans and animals.[2] Depending on the pathogen, which can be a virus or intestinal parasite, the host may be infectious and able to transmit the pathogen without ever developing symptoms;[3][4] such a host is called an asymptomatic carrier.[3] Many pathogens, including HIV, typhoid fever, and coronaviruses such as COVID-19 spread in their host populations through subclinical infection.[3][4][5]
Not all hosts of asymptomatic subclinical infections will become asymptomatic carriers. For example, hosts of Mycobacterium tuberculosis bacteria will only develop active tuberculosis in approximately one-tenth of cases;[6] the majority of those infected by Mtb bacteria have latent tuberculosis, a non-infectious type of tuberculosis that does not produce symptoms in individuals with sufficient immune responses.[7]
Because subclinical infections often occur without eventual overt sign, in some cases their presence is only identified by microbiological culture or DNA techniques such as polymerase chain reaction (PCR) tests.[8][9]
^Porta, Miquel (2014). Oxford dictionary of epidemiology (6th ed.). Oxford: Oxford University Press. ISBN978-0-19-997672-0.[page needed]