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Chapter 1: Infectious and parasitic diseases (A00–B99)

Infections caused by bacteria, viruses, fungi and parasites — from gastroenteritis and tuberculosis to sepsis, sexually transmitted infections and HIV — together with codes identifying the organisms that cause them.

Chapter 1 spans codes A00–B99 in the FY2027 ICD-10-CM tabular list. This guide explains 26 of its most common codes — select a code for its official description, a plain-English explanation and a note on when the code is used.

ICD-10 Chapter 1: Infectious & Parasitic Diseases — illustrative image

Codes in this chapter

CodeOfficial descriptionIn plain words
A04.72Enterocolitis due to Clostridium difficile, not specified as recurrentA bowel infection caused by the bacterium Clostridioides difficile, often triggered when antibiotics disturb the normal gut bacteria.
A08.4Viral intestinal infection, unspecifiedA gut infection caused by a virus, producing vomiting and watery diarrhoea, usually settling within a few days.
A09Infectious gastroenteritis and colitis, unspecifiedInflammation of the stomach and intestines, or of the colon, caused by an infection, with diarrhoea as the main symptom.
A15.0Tuberculosis of lungTuberculosis infection of the lungs, caused by Mycobacterium tuberculosis, spread through the air from people with active disease.
A41.51Sepsis due to Escherichia coli [E. coli]Sepsis caused by the common gut bacterium Escherichia coli (E. coli).
A41.9Sepsis, unspecified organismSepsis is the body's overwhelming, life-threatening response to an infection, which can lead to organ failure.
A53.9Syphilis, unspecifiedSyphilis is a sexually transmitted bacterial infection that progresses through stages if untreated.
A54.9Gonococcal infection, unspecifiedGonorrhoea is a sexually transmitted bacterial infection of the genital tract, rectum or throat.
A59.9Trichomoniasis, unspecifiedTrichomoniasis is a common sexually transmitted infection caused by a tiny parasite.
A60.9Anogenital herpesviral infection, unspecifiedHerpes simplex virus infection around the genitals or anus, causing recurrent painful blisters or sores.
A63.0Anogenital (venereal) wartsGenital or anal warts caused by human papillomavirus (HPV), a sexually transmitted virus.
A69.20Lyme disease, unspecifiedLyme disease is an infection spread by tick bites, often starting with an expanding round rash.
A74.9Chlamydial infection, unspecifiedChlamydia is a very common sexually transmitted bacterial infection that often causes no symptoms.
B00.9Herpesviral infection, unspecifiedInfection with herpes simplex virus, the virus behind cold sores and genital herpes.
B02.9Zoster without complicationsShingles is a painful blistering rash caused by reactivation of the chickenpox virus in a nerve root.
B08.4Enteroviral vesicular stomatitis with exanthemHand, foot and mouth disease is a common childhood viral illness with sores in the mouth and a rash on the hands and feet.
B18.2Chronic viral hepatitis CLong-term (chronic) infection of the liver with the hepatitis C virus.
B20Human immunodeficiency virus [HIV] diseaseDisease caused by the human immunodeficiency virus (HIV), which weakens the immune system over time.
B34.9Viral infection, unspecifiedAn illness caused by a virus, where tests do not identify a specific virus or site.
B35.1Tinea unguiumA fungal infection of the nails, usually toenails, making them thickened, discoloured and crumbly.
B35.4Tinea corporisRingworm of the body: a fungal skin infection causing itchy, ring-shaped patches.
B37.0Candidal stomatitisOral thrush: a yeast (Candida) infection causing white patches inside the mouth.
B85.0Pediculosis due to Pediculus humanus capitisHead lice infestation: tiny insects living on the scalp, spread by head-to-head contact, common in schoolchildren.
B86ScabiesScabies is an intensely itchy skin infestation caused by a microscopic mite that burrows into the skin.
B96.81Helicobacter pylori [H. pylori] as the cause of diseases classified elsewhereHelicobacter pylori is a bacterium that lives in the stomach lining and is linked to stomach ulcers and gastritis.
B97.7Papillomavirus as the cause of diseases classified elsewhereHuman papillomavirus (HPV) is a very common virus; some types cause warts and others are linked to cervical and other cancers.

This guide explains ICD-10-CM codes in plain language for patients, students and other readers. It is educational information only — not coding, billing or medical advice. The code recorded for any real encounter is assigned from the full clinical documentation by the treating team and qualified coders, and only they can decide which code fits a specific case.

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