Subclinical infection

A subclinical infection (sometimes called a preinfection) 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.  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: 

 Baylisascaris procyonis
 Bordetella pertussis (Pertussis or whooping cough)
 Chlamydia pneumoniae
 Chlamydia trachomatis (Chlamydia)
 Clostridium difficile
 Cyclospora cayetanensis
 Dengue virus
 Dientamoeba fragilis
 Entamoeba histolytica
 enterotoxigenic Escherichia coli
 Epstein-Barr virus
 Group A streptococcal infection
 Helicobacter pylori
 Herpes simplex (oral herpes, genital herpes, etc.)
 HIV-1 (AIDS)
 Legionella pneumophila (Legionnaires' disease)
 measles viruses
 Mycobacterium leprae (leprosy)
 Mycobacterium tuberculosis (tuberculosis)
 Neisseria gonorrhoeae (gonorrhoea)
 Neisseria meningitidis (Meningitis) 
 nontyphoidal Salmonella
 noroviruses
 Poliovirus (Poliomyelitis)
 Plasmodium (Malaria)
 rhinoviruses (Common cold)
 Salmonella enterica serovar Typhi (Typhoid fever)
 Staphylococcus aureus
 Streptococcus pneumoniae (Bacterial pneumonia)
 Treponema pallidum (syphilis)

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

References

Further reading

Category:Human diseases and disorders
Category:Epidemiology
Category:Infectious diseases
Category:Medical terminology
Symptoms