In infectious disease, subclinical infections refer to an infection occurring in the body — such as a pathogenic bacteria colonizing or a virus actively reproducing — but at low enough levels that no physical symptoms occur. Other terms that can refer to subclinical infections include asymptomatic infections and inapparent infections. A person with a subclinical infection has an infection but does not have the disease since disease refers to both the infection and the associated signs and symptoms of infection.
Deeper Dive
Subclinical infections present epidemiological challenges to tracking a disease that’s communicable between people because it’s harder to identify who has the infection and is actively shedding the pathogen. For example, an estimated 70% of people with a polio infection have subclinical infections — there’s no way to know they have polio unless all healthy-appearing individuals are tested for the virus. But individuals with an active poliovirus infection can still transmit the virus to others, even though they do not have disease. A number of human viruses have high rates of subclinical infection; another is West Nile virus, which is subclinical/asymptomatic in about 80% of cases.
Subclinical infection is different from an infection that is still in the incubation period, before symptoms start. COVID-19, for example, can be transmitted during the incubation period or can be transmitted when someone is asymptomatic. But transmission risk during the incubation period is usually higher than during an asymptomatic infection because someone asymptomatic typically has a lower viral load (total number of virus particles in the body) and therefore will usually shed fewer viruses in fluids, breath, and feces.
How infectious a person is with a subclinical infection varies according to the pathogen. One of the most famous examples of the trouble subclinical infections can cause is the case of “Typhoid Mary,” who was an asymptomatic carrier of the Salmonella typhi bacteria that causes typhoid fever. She was believed to be the primary driver of a typhoid outbreak in New York City in 1907 and is thought to have infected an estimated 50-120 people.