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]

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

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]



In humans


Many pathogens are transmitted through their host populations by hosts with few or no symptoms, including sexually transmitted infections such as syphilis and genital warts.[10] In other cases, a host may develop more symptoms as the infection progresses beyond its incubation period.[3][7] 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. This prevents accurate modeling of its transmissibility.[11][12]

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.[13] While HPAI viruses have a very high mortality rate for chickens,[14] LPAI viruses are very mild and produce few, if any symptoms; outbreaks in a flock may go undetected without ongoing testing.[14]

Wild ducks and other waterfowl are asymptomatic carriers of avian influenza, notably HPAI, and can be infected without showing signs of illness.[13][15] The prevalence of subclinical HPAI infection in waterfowl has contributed to the international outbreak of highly lethal H5N8 virus that began in early 2020.[13][16]

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:

See also



  1. ^ Porta, Miquel (2014). Oxford dictionary of epidemiology (6th ed.). Oxford: Oxford University Press. ISBN 978-0-19-997672-0.[page needed]
  2. ^ Dimijian, Gregory G. (January 2000). "Pathogens and Parasites: Strategies and Challenges". Baylor University Medical Center Proceedings. 13 (1): 19–29. doi:10.1080/08998280.2000.11927638. PMC 1312209. PMID 16389321.
  3. ^ a b c d "Principles of Epidemiology | Lesson 1 - Section 9". www.cdc.gov. 2021-12-20. Retrieved 2023-01-26.
  4. ^ a b Bogoch, Isaac (16 May 2020). "The Term 'Asymptomatic' Is More Complicated Than It Seems When It Comes To Coronavirus Infections". Forbes.
  5. ^ "Questions and Answers | Typhoid Fever | CDC". www.cdc.gov. 2020-11-16. Archived from the original on 2016-06-08. Retrieved 2023-01-26.
  6. ^ "Tuberculosis (TB) Detailed Explanation". National Institute of Allergy and Infectious Diseases. 2009-03-06. Archived from the original on 2010-04-11. Retrieved 2023-01-26.
  7. ^ a b Drain, Paul K.; Bajema, Kristina L.; Dowdy, David; Dheda, Keertan; Naidoo, Kogieleum; Schumacher, Samuel G.; Ma, Shuyi; Meermeier, Erin; Lewinsohn, David M.; Sherman, David R. (2018-07-18). "Incipient and Subclinical Tuberculosis: a Clinical Review of Early Stages and Progression of Infection". Clinical Microbiology Reviews. 31 (4): e00021–18. doi:10.1128/CMR.00021-18. ISSN 0893-8512. PMC 6148193. PMID 30021818.
  8. ^ Yang, S.; Rothman, R. E. (2004). "PCR-based diagnostics for infectious diseases: Uses, limitations, and future applications in acute-care settings". The Lancet. Infectious Diseases. 4 (6): 337–348. doi:10.1016/S1473-3099(04)01044-8. PMC 7106425. PMID 15172342.
  9. ^ Miller, J. M.; Binnicker, M. J.; Campbell, S.; Carroll, K. C.; Chapin, K. C.; Gilligan, P. H.; Gonzalez, M. D.; Jerris, R. C.; Kehl, S. C.; Patel, R.; Pritt, B. S.; Richter, S. S.; Robinson-Dunn, B.; Schwartzman, J. D.; Snyder, J. W.; Telford S III; Theel, E. S.; Thomson Jr, R. B.; Weinstein, M. P.; Yao, J. D. (2018). "A Guide to Utilization of the Microbiology Laboratory for Diagnosis of Infectious Diseases: 2018 Update by the Infectious Diseases Society of America and the American Society for Microbiology". Clinical Infectious Diseases. 67 (6): e1–e94. doi:10.1093/cid/ciy381. PMC 7108105. PMID 29955859.
  10. ^ Mayer, K. H.; Collis, T. K.; Celum, C. L. (15 February 2001). "The Clinical Manifestations and Treatment of Sexually Transmitted Diseases in Human Immunodeficiency Virus-Positive Men". Clinical Infectious Diseases. 32 (4): 611–622. doi:10.1086/318722. PMID 11181126.
  11. ^ Van Seventer, Jean Maguire; Hochberg, Natasha S. (2017). "Principles of Infectious Diseases: Transmission, Diagnosis, Prevention, and Control". International Encyclopedia of Public Health. pp. 22–39. doi:10.1016/B978-0-12-803678-5.00516-6. ISBN 978-0-12-803708-9. PMC 7150340. S2CID 78523832.
  12. ^ Ansumali, S.; Kaushal, S.; Kumar, A.; Prakash, M. K.; Vidyasagar, M. (2020). "Modelling a pandemic with asymptomatic patients, impact of lockdown and herd immunity, with applications to SARS-CoV-2". Annual Reviews in Control. 50: 432–447. doi:10.1016/j.arcontrol.2020.10.003. PMC 7546280. PMID 33071595.
  13. ^ a b c CDC (2022-03-09). "Avian Influenza in Birds". Centers for Disease Control and Prevention. Retrieved 2023-01-26.
  14. ^ a b "Avian influenza: assessing the pandemic threat" (PDF). WHO. 2005. Retrieved 2023-01-26.
  15. ^ "Avian influenza in Canada". Ducks Unlimited Canada. Retrieved 2023-01-26.
  16. ^ Government of Canada, Canadian Food Inspection Agency (2021-02-26). "Protect your flock from bird flu". www.inspection.gc.ca. Retrieved 2023-01-26.
  17. ^ Weinstein, Sara B.; Lake, Camille M.; Chastain, Holly M.; Fisk, David; Handali, Sukwan; Kahn, Philip L.; Montgomery, Susan P.; Wilkins, Patricia P.; Kuris, Armand M.; Lafferty, Kevin D. (August 2017). "Seroprevalence of Baylisascaris procyonis Infection among Humans, Santa Barbara County, California, USA, 2014–2016". Emerging Infectious Diseases. 23 (8): 1397–1399. doi:10.3201/eid2308.170222. PMC 5547801. PMID 28726612.
  18. ^ Klement E, Grotto I, Srugo I, Orr N, Gilad J, Cohent D (March 2005). "Pertussis in soldiers, Israel". Emerging Infect. Dis. 11 (3): 506–8. doi:10.3201/eid1103.040672. PMC 3298243. PMID 15789494.
  19. ^ Müller J, Møller DS, Kjaer M, Nyvad O, Larsen NA, Pedersen EB (2003). "Chlamydia pneumoniae DNA in peripheral blood mononuclear cells in healthy control subjects and patients with diabetes mellitus, acute coronary syndrome, stroke, and arterial hypertension". Scand. J. Infect. Dis. 35 (10): 704–12. doi:10.1080/00365540310016538. PMID 14606608. S2CID 43211363.
  20. ^ a b Cecil, Jane A.; Howell, M. Rene; Tawes, Jennifer J.; Gaydos, Joel C.; McKee, Jr., Kelly T.; Quinn, Thomas C.; Gaydos, Charlotte A. (November 2001). "Features of Chlamydia trachomatis and Neisseria gonorrhoeae Infection in Male Army Recruits". The Journal of Infectious Diseases. 184 (9): 1216–1219. doi:10.1086/323662. PMID 11598849.
  21. ^ a b Korenromp, Eline L; Sudaryo, Mondastri K; de Vlas, Sake J; Gray, Ronald H; Sewankambo, Nelson K; Serwadda, David; Wawer, Maria J; Habbema, J Dik F (1 February 2002). "What proportion of episodes of gonorrhoea and chlamydia becomes symptomatic?". International Journal of STD & AIDS. 13 (2): 91–101. doi:10.1258/0956462021924712. PMID 11839163. S2CID 33926189.
  22. ^ Sutton TL, Martinko T, Hale S, Fairchok MP (December 2003). "Prevalence and high rate of asymptomatic infection of Chlamydia trachomatis in male college Reserve Officer Training Corps cadets". Sex Transm Dis. 30 (12): 901–4. doi:10.1097/01.OLQ.0000091136.14932.8B. PMID 14646638. S2CID 43394485.
  23. ^ Rivera EV, Woods S (2003). "Prevalence of asymptomatic Clostridium difficile colonization in a nursing home population: a cross-sectional study". J Gend Specif Med. 6 (2): 27–30. PMID 12813999.
  24. ^ Chacin-Bonilla L, Mejia de Young M, Estevez J (March 2003). "Prevalence and pathogenic role of Cyclospora cayetanensis in a Venezuelan community". Am. J. Trop. Med. Hyg. 68 (3): 304–6. doi:10.4269/ajtmh.2003.68.304. PMID 12685635.
  25. ^ Burke, Donald S.; Scott, Robert McN.; Johnson, David E.; Nisalak, Ananda (1 January 1988). "A Prospective Study of Dengue Infections in Bangkok". The American Journal of Tropical Medicine and Hygiene. 38 (1): 172–180. doi:10.4269/ajtmh.1988.38.172. PMID 3341519. INIST 7572063.
  26. ^ Peek R, Reedeker FR, van Gool T (February 2004). "Direct Amplification and Genotyping of Dientamoeba fragilis from Human Stool Specimens". J. Clin. Microbiol. 42 (2): 631–5. doi:10.1128/JCM.42.2.631-635.2004. PMC 344490. PMID 14766828.
  27. ^ Blessmann, Joerg; Ali, Ibne Karim M.; Ton Nu, Phuong A.; Dinh, Binh T.; Ngo Viet, Tram Q.; Le Van, An; Clark, C. Graham; Tannich, Egbert (October 2003). "Longitudinal Study of Intestinal Entamoeba histolytica Infections in Asymptomatic Adult Carriers". Journal of Clinical Microbiology. 41 (10): 4745–4750. doi:10.1128/JCM.41.10.4745-4750.2003. PMC 294961. PMID 14532214.
  28. ^ Wennerås, Christine; Erling, Valdemar (2004). "Prevalence of Enterotoxigenic Escherichia coli-associated Diarrhoea and Carrier State in the Developing World". Journal of Health, Population and Nutrition. 22 (4): 370–382. JSTOR 23499155. PMID 15663170.
  29. ^ Pegtel DM, Middeldorp J, Thorley-Lawson DA (November 2004). "Epstein-Barr Virus Infection in Ex Vivo Tonsil Epithelial Cell Cultures of Asymptomatic Carriers". J. Virol. 78 (22): 12613–24. doi:10.1128/JVI.78.22.12613-12624.2004. PMC 525079. PMID 15507648.
  30. ^ Ozturk CE, Yavuz T, Kaya D, Yucel M (December 2004). "The rate of asymptomatic throat carriage of group A Streptococcus in school children and associated ASO titers in Duzce, Turkey". Jpn. J. Infect. Dis. 57 (6): 271–2. hdl:20.500.12491/5541. PMID 15623954.
  31. ^ Kul S, Sert B, Sari A, et al. (September 2008). "Effect of subclinical Helicobacter pylori infection on gastric wall thickness: multislice CT evaluation". Diagn Interv Radiol. 14 (3): 138–42. PMID 18814135.
  32. ^ Wald A, Zeh J, Selke S, Ashley RL, Corey L (September 1995). "Virologic characteristics of subclinical and symptomatic genital herpes infections". N. Engl. J. Med. 333 (12): 770–5. doi:10.1056/NEJM199509213331205. PMID 7643884.
  33. ^ Mummidi, Srinivas; Ahuja, Seema S.; Gonalez, Enrique; Anderson, Stephanie A.; Santiago, Elvin N.; Stephan, Kevin T.; Craig, Fiona E.; O'Connell, Peter; Tryon, Victor; Clark, Robert A.; Dolan, Matthew J.; Ahuja, Sunil K. (July 1998). "Genealogy of the CCR5 locus and chemokine system gene variants associated with altered rates of HIV-1 disease progression". Nature Medicine. 4 (7): 786–793. doi:10.1038/nm0798-786. PMID 9662369. S2CID 30305043.
  34. ^ Flournoy DJ, Guthrie PJ, Lawrence CH, Silberg SL, Beaver S (January 1990). "Incidence of Legionella pneumophila infections among Oklahoma pulmonary disease patients". J Natl Med Assoc. 82 (1): 25–9. PMC 2625929. PMID 2304095.
  35. ^ Anlar, B; Ayhan, A; Hotta, H; Itoh, M; Engin, D; Barun, S; Koseoglu, O (August 2002). "Measles Virus RNA in Tonsils of Asymptomatic Children". Journal of Paediatrics and Child Health. 38 (4): 424–425. doi:10.1046/j.1440-1754.2002.t01-1-00029.x. PMID 12174013. S2CID 40754266.
  36. ^ Beyene, D.; Aseffa, A.; Harboe, M.; Kidane, D.; Macdonald, M.; Klatser, P. R.; Bjune, G. A.; Smith, W. C. S. (October 2003). "Nasal carriage of Mycobacterium leprae DNA in healthy individuals in Lega Robi village, Ethiopia". Epidemiology and Infection. 131 (2): 841–848. doi:10.1017/s0950268803001079. PMC 2870027. PMID 14596524.
  37. ^ Dye C, Scheele S, Dolin P, Pathania V, Raviglione MC (August 1999). "Consensus statement. Global burden of tuberculosis: estimated incidence, prevalence, and mortality by country. WHO Global Surveillance and Monitoring Project". JAMA. 282 (7): 677–86. doi:10.1001/jama.282.7.677. PMID 10517722.
  38. ^ Yazdankhah SP, Caugant DA (September 2004). "Neisseria meningitidis: an overview of the carriage state". J. Med. Microbiol. 53 (Pt 9): 821–32. CiteSeerX doi:10.1099/jmm.0.45529-0. PMID 15314188. INIST 16081902.
  39. ^ Sirinavin S, Pokawattana L, Bangtrakulnondh A (June 2004). "Duration of nontyphoidal Salmonella carriage in asymptomatic adults". Clin. Infect. Dis. 38 (11): 1644–5. doi:10.1086/421027. PMID 15156460.
  40. ^ Gallimore CI, Cubitt D, du Plessis N, Gray JJ (May 2004). "Asymptomatic and Symptomatic Excretion of Noroviruses during a Hospital Outbreak of Gastroenteritis". J. Clin. Microbiol. 42 (5): 2271–4. doi:10.1128/JCM.42.5.2271-2274.2004. PMC 404621. PMID 15131210.
  41. ^ van Benten I, Koopman L, Niesters B, et al. (October 2003). "Predominance of rhinovirus in the nose of symptomatic and asymptomatic infants". Pediatr Allergy Immunol. 14 (5): 363–70. doi:10.1034/j.1399-3038.2003.00064.x. PMC 7168036. PMID 14641606.
  42. ^ Parry CM, Hien TT, Dougan G, White NJ, Farrar JJ (November 2002). "Typhoid fever". N. Engl. J. Med. 347 (22): 1770–82. doi:10.1056/NEJMra020201. PMID 12456854. S2CID 5059045.
  43. ^ Camilla Rothe; Mirjam Schunk; Peter Sothmann; et al. (30 January 2020). "Transmission of 2019-nCoV Infection from an Asymptomatic Contact in Germany". The New England Journal of Medicine. 382 (10): 970–971. doi:10.1056/NEJMC2001468. ISSN 0028-4793. PMC 7120970. PMID 32003551. S2CID 210982826. Wikidata Q84106604.
  44. ^ Fung, Sin-Yee; Yuen, Kit-San; Ye, Zi-Wei; Chan, Chi-Ping; Jin, Dong-Yan (1 January 2020). "A tug-of-war between severe acute respiratory syndrome coronavirus 2 and host antiviral defence: lessons from other pathogenic viruses". Emerging Microbes & Infections. 9 (1): 558–570. doi:10.1080/22221751.2020.1736644. PMC 7103735. PMID 32172672.
  45. ^ Kenner, Julie; O'Connor, Tasha; Piantanida, Nicholas; Fishbain, Joel; Eberly, Bardwell; Viscount, Helen; Uyehara, Catherine; Hospenthal, Duane (June 2003). "Rates of Carriage of Methicillin-Resistant and Methicillin-Susceptible Staphylococcus aureus in an Outpatient Population". Infection Control & Hospital Epidemiology. 24 (6): 439–444. doi:10.1086/502229. PMID 12828322. S2CID 27801815.
  46. ^ Malfroot A, Verhaegen J, Dubru JM, Van Kerschaver E, Leyman S (September 2004). "A cross-sectional survey of the prevalence of Streptococcus pneumoniae nasopharyngeal carriage in Belgian infants attending day care centres". Clin. Microbiol. Infect. 10 (9): 797–803. doi:10.1111/j.1198-743X.2004.00926.x. PMID 15355410.
  47. ^ Singh AE, Romanowski B (April 1999). "Syphilis: Review with Emphasis on Clinical, Epidemiologic, and Some Biologic Features". Clin. Microbiol. Rev. 12 (2): 187–209. doi:10.1128/CMR.12.2.187. PMC 88914. PMID 10194456.

Further reading