A subclinical infection—sometimes called a preinfection or inapparent infection—is an

infection An infection is the invasion of tissue (biology), tissues by pathogens, their multiplication, and the reaction of host (biology), host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmis ...

by a

pathogen In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory of d ...

that causes few or no signs or symptoms of infection in the host. Subclinical infections can occur in both humans and animals. Depending on the pathogen, which can be a

virus A virus is a submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are ...

or intestinal parasite, the host may be infectious and able to transmit the pathogen without ever developing symptoms; such a host is called an asymptomatic carrier. Many pathogens, including HIV, typhoid fever, and

coronavirus Coronaviruses are a group of related RNA viruses that cause diseases in mammals and birds. In humans and birds, they cause respiratory tract infections that can range from mild to lethal. Mild illnesses in humans include some cases of the comm ...

es such as

COVID-19 Coronavirus disease 2019 (COVID-19) is a contagious disease caused by the coronavirus SARS-CoV-2. In January 2020, the disease spread worldwide, resulting in the COVID-19 pandemic. The symptoms of COVID‑19 can vary but often include fever ...

spread in their host populations through subclinical infection. Not all hosts of asymptomatic subclinical infections will become asymptomatic carriers. For example, hosts of ''

Mycobacterium tuberculosis ''Mycobacterium tuberculosis'' (M. tb), also known as Koch's bacillus, is a species of pathogenic bacteria in the family Mycobacteriaceae and the causative agent of tuberculosis. First discovered in 1882 by Robert Koch, ''M. tuberculosis'' ha ...

'' bacteria will only develop active

tuberculosis Tuberculosis (TB), also known colloquially as the "white death", or historically as consumption, is a contagious disease usually caused by ''Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but it can al ...

in approximately one-tenth of cases; 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. Because subclinical infections often occur without eventual overt sign, in some cases their presence is only identified by

microbiological culture A microbiological culture, or microbial culture, is a method of multiplying microorganism, microbial organisms by letting them reproduce in predetermined culture medium under controlled laboratory conditions. Microbial cultures are foundational a ...

or DNA techniques such as

polymerase chain reaction The polymerase chain reaction (PCR) is a method widely used to make millions to billions of copies of a specific DNA sample rapidly, allowing scientists to amplify a very small sample of DNA (or a part of it) sufficiently to enable detailed st ...

(PCR) tests.

Transmission

In humans

Many pathogens are transmitted through their host populations by hosts with few or no symptoms, including

sexually transmitted infection A sexually transmitted infection (STI), also referred to as a sexually transmitted disease (STD) and the older term venereal disease (VD), is an infection that is Transmission (medicine), spread by Human sexual activity, sexual activity, e ...

s such as

syphilis Syphilis () is a sexually transmitted infection caused by the bacterium ''Treponema pallidum'' subspecies ''pallidum''. The signs and symptoms depend on the stage it presents: primary, secondary, latent syphilis, latent or tertiary. The prim ...

and genital warts. In other cases, a host may develop more symptoms as the infection progresses beyond its incubation period. These hosts create a natural reservoir of individuals that can transmit a pathogen to other individuals. Because cases often do not come to clinical attention, health statistics frequently are unable to measure the true prevalence of an infection in a

population Population is a set of humans or other organisms in a given region or area. Governments conduct a census to quantify the resident population size within a given jurisdiction. The term is also applied to non-human animals, microorganisms, and pl ...

. This prevents accurate modeling of its transmissibility.

In animals

Some animal pathogens are also transmitted through subclinical infections. The A(H5) and A(H7) strains of avian influenza are divided into two categories: low pathogenicity avian influenza (LPAI) viruses and highly pathogenic avian influenza (HPAI) viruses. While HPAI viruses have a very high mortality rate for

chicken The chicken (''Gallus gallus domesticus'') is a domesticated subspecies of the red junglefowl (''Gallus gallus''), originally native to Southeast Asia. It was first domesticated around 8,000 years ago and is now one of the most common and w ...

s, LPAI viruses are very mild and produce few, if any symptoms; outbreaks in a flock may go undetected without ongoing testing. Wild

duck Duck is the common name for numerous species of waterfowl in the family (biology), family Anatidae. Ducks are generally smaller and shorter-necked than swans and goose, geese, which are members of the same family. Divided among several subfam ...

s and other waterfowl are asymptomatic carriers of avian influenza, notably HPAI, and can be infected without showing signs of illness. The prevalence of subclinical HPAI infection in waterfowl has contributed to the international outbreak of highly lethal H5N8 virus that began in early 2020.

Pathogens known to cause subclinical infection

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 ''Baylisascaris'' is a genus of nematode, roundworms that infect more than fifty animal species. Life cycle ''Baylisascaris'' eggs are passed in feces and become active within a month. They can remain viable in the environment for years, withs ...

'' * '' Bordetella pertussis'' ( Pertussis or whooping cough) * '' Chlamydia pneumoniae'' * ''

Chlamydia trachomatis ''Chlamydia trachomatis'' () is a Gram-negative, Anaerobic organism, anaerobic bacterium responsible for Chlamydia infection, chlamydia and trachoma. ''C. trachomatis'' exists in two forms, an extracellular infectious elementary body (EB) and an ...

'' ( Chlamydia) * '' Clostridioides difficile'' * '' Cyclospora cayetanensis'' * Dengue virus * '' Dientamoeba fragilis'' * '' Entamoeba histolytica'' * Enterotoxigenic Escherichia coli * Epstein–Barr virus *

Group A streptococcal infection Group A streptococcal infections are a number of infections with ''Streptococcus pyogenes'', a group A streptococcus (GAS). ''S. pyogenes'' is a species of beta-hemolytic Gram-positive bacteria that is responsible for a wide range of infectio ...

* '' Helicobacter pylori'' * ''

Herpes simplex Herpes simplex, often known simply as herpes, is a viral disease, viral infection caused by the herpes simplex virus. Herpes infections are categorized by the area of the body that is infected. The two major types of herpes are Cold sore, ora ...

'' ( oral herpes, genital herpes, etc.) * HIV-1 (

HIV/AIDS The HIV, human immunodeficiency virus (HIV) is a retrovirus that attacks the immune system. Without treatment, it can lead to a spectrum of conditions including acquired immunodeficiency syndrome (AIDS). It is a Preventive healthcare, pr ...

) * ''

Influenza Influenza, commonly known as the flu, is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These sympto ...

'' (strains) * ''

Legionella pneumophila ''Legionella pneumophila'', the primary causative agent for Legionnaires' disease, Legionnaire's disease, is an Aerobic organism, aerobic, pleomorphic, Flagellum, flagellated, non-spore-forming, Gram-negative bacteria, Gram-negative bacterium. ' ...

'' (

Legionnaires' disease Legionnaires' disease is a form of atypical pneumonia caused by any species of ''Legionella'' bacteria, quite often ''Legionella pneumophila''. Signs and symptoms include cough, shortness of breath, high fever, myalgia, muscle pains, and headach ...

) *

Measles Measles (probably from Middle Dutch or Middle High German ''masel(e)'', meaning "blemish, blood blister") is a highly contagious, Vaccine-preventable diseases, vaccine-preventable infectious disease caused by Measles morbillivirus, measles v ...

viruses * '' Mycobacterium leprae'' (

leprosy Leprosy, also known as Hansen's disease (HD), is a Chronic condition, long-term infection by the bacteria ''Mycobacterium leprae'' or ''Mycobacterium lepromatosis''. Infection can lead to damage of the Peripheral nervous system, nerves, respir ...

) * ''

'' (

) * '' Neisseria gonorrhoeae'' ( gonorrhoea) * '' Neisseria meningitidis'' (

Meningitis Meningitis is acute or chronic inflammation of the protective membranes covering the brain and spinal cord, collectively called the meninges. The most common symptoms are fever, intense headache, vomiting and neck stiffness and occasion ...

) * Nontyphoidal

Salmonella ''Salmonella'' is a genus of bacillus (shape), rod-shaped, (bacillus) Gram-negative bacteria of the family Enterobacteriaceae. The two known species of ''Salmonella'' are ''Salmonella enterica'' and ''Salmonella bongori''. ''S. enterica'' ...

* Noroviruses * Poliovirus ( Poliomyelitis) *

Plasmodium ''Plasmodium'' is a genus of unicellular eukaryotes that are obligate parasites of vertebrates and insects. The life cycles of ''Plasmodium'' species involve development in a Hematophagy, blood-feeding insect host (biology), host which then inj ...

(

Malaria Malaria is a Mosquito-borne disease, mosquito-borne infectious disease that affects vertebrates and ''Anopheles'' mosquitoes. Human malaria causes Signs and symptoms, symptoms that typically include fever, Fatigue (medical), fatigue, vomitin ...

) * Rabies lyssavirus ( Rabies) *

Rhinovirus The rhinovirus (from the "nose", , romanized: "of the nose", and the ) is a Positive-sense single stranded RNA virus, positive-sense, single-stranded RNA virus belonging to the genus ''Enterovirus'' in the family ''Picornaviridae''. Rhinoviru ...

es (

Common cold The common cold, or the cold, is a virus, viral infectious disease of the upper respiratory tract that primarily affects the Respiratory epithelium, respiratory mucosa of the human nose, nose, throat, Paranasal sinuses, sinuses, and larynx. ...

) * '' Salmonella enterica serovar Typhi'' ( Typhoid fever) * ''

SARS-CoV-2 Severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2) is a strain of coronavirus that causes COVID-19, the respiratory illness responsible for the COVID-19 pandemic. The virus previously had the Novel coronavirus, provisional nam ...

'' (

) and other coronaviruses * ''

Staphylococcus aureus ''Staphylococcus aureus'' is a Gram-positive spherically shaped bacterium, a member of the Bacillota, and is a usual member of the microbiota of the body, frequently found in the upper respiratory tract and on the skin. It is often posi ...

'' * '' Streptococcus pneumoniae'' ( Bacterial pneumonia) * '' Treponema pallidum'' (

)

Transmission

In humans

In animals

Pathogens known to cause subclinical infection

See also

References

Further reading