Subclinical Infection

Typhoid Mary, pictured above in a 1909 tabloid, was a famous case of a subclinical infection of Salmonella enterica serovar.

A subclinical infection — sometimes called a preinfection or inapparent infection — is an infection that, being subclinical, is nearly or completely asymptomatic (no signs or symptoms). A subclinically infected person is thus an asymptomatic carrier of a microbe, intestinal parasite, or virus that usually is a pathogen causing illness, at least in some individuals. Many pathogens spread by being silently carried in this way by some of their host population. Such infections occur both in humans and nonhuman animals.

An example of an asymptomatic infection is a mild common cold that is not noticed by the infected individual. Since subclinical infections often occur without eventual overt sign, their existence is only identified by microbiological culture or DNA techniques such as polymerase chain reaction.

Infection transmission/signs

An individual may only develop signs of an infection after a period of subclinical infection, a duration that is called the incubation period. This is the case, for example, for subclinical sexually transmitted diseases such as AIDS and genital warts. It is thought that individuals with such subclinical infections, and those that never develop overt illness, creates a reserve of individuals that can transmit an infectious agent to infect other individuals. Because such cases of infections do not come to clinical attention, health statistics can often fail to measure the true prevalence of an infection in a population, and this prevents the accurate modeling of its infectious transmission.

Types of subclinical infections

The following pathogens (together with their symptomatic illnesses) are known to be carried asymptomatically, often in a large percentage of the potential host population:

Host tolerance

Fever and sickness behavior and other signs of infection are often taken to be due to them. However, they are evolved physiological and behavioral responses of the host to clear itself of the infection. Instead of incurring the costs of deploying these evolved responses to infections, the body opts to tolerate an infection as an alternative to seeking to control or remove the infecting pathogen.

Subclinical infections are important since they allow infections to spread from a reserve of carriers. They also can cause clinical problems unrelated to the direct issue of infection. For example, in the case of urinary tract infections in women, this infection may cause preterm delivery if the person becomes pregnant without proper treatment.

See also