Classifying Infectious and Parasitic Diseases Under ICD-11

Every disease that gets diagnosed, reported, or studied around the world needs a common language. Without one, a case of typhoid in Patna and a case of typhoid in Paris would be hard to compare. That’s where the World Health Organization’s International Classification of Diseases (ICD) steps in, and its latest edition, ICD-11, dedicates an entire opening chapter to infectious and parasitic diseases – the conditions that have shaped human history more than almost any other.

What are infectious and parasitic diseases? 🔗

Infectious diseases are illnesses caused by pathogens – living agents that invade the human body, multiply, and trigger sickness. ICD-11’s first chapter explicitly groups together conditions caused by pathogenic organisms or microorganisms, such as bacteria, viruses, parasites or fungi. The chapter code range starts with 1A00 and runs through several hundred sub-codes, covering everything from a mild bout of food poisoning to life-threatening conditions like tuberculosis and HIV.

The four main categories of pathogens behave very differently:

Bacteria 🔗

Single-celled organisms that can multiply on their own, both inside and outside the body. Some are friendly (the bacteria in curd, for example), but pathogenic ones cause illnesses like tuberculosis, cholera, and typhoid. Antibiotics work against bacteria – but only when used correctly.

Viruses 🔗

Viruses are not technically alive on their own. They need a host cell to replicate. They cause diseases like dengue, influenza, hepatitis, measles, and COVID-19. Antibiotics do not work on viruses; antivirals and vaccines are the primary tools.

Fungi 🔗

Fungi cause infections ranging from common ringworm and athlete’s foot to serious invasive infections in people with weakened immunity. The “black fungus” cases reported during the COVID-19 second wave were a sharp reminder of how dangerous fungal infections can become.

Parasites 🔗

Parasites are organisms that live on or inside a host and feed off it. They cause malaria (protozoa), worm infestations (helminths), and conditions like leishmaniasis or kala-azar. The ICD-11 sub-block for parasitic diseases includes malaria, non-intestinal protozoal diseases and helminthiases.

How ICD-11 organises this chapter 🔗

The biggest shift in ICD-11, which came into effect on 1 January 2022 after being adopted by the 72nd World Health Assembly in 2019, is its digital-first design. Each entity has a unique code, but the classification also allows the same disease to appear in multiple meaningful places. Pneumonia, for instance, is a lung infection – but it is also an infectious disease – so it shows up as a “grey node” in both chapters without being counted twice.

Within Chapter 1, diseases are grouped in a way that helps both clinicians and statisticians make sense of patterns. Some of the major sub-blocks include:

Gastroenteritis or colitis of infectious origin (1A00-1A40) 🔗

This covers bacterial intestinal infections like cholera and shigellosis, viral intestinal infections like rotavirus, and protozoal infections like amoebiasis. In simpler terms – most cases of diarrhoea and food poisoning that send people to a hospital fall here. Diarrhoeal diseases remain a leading cause of childhood mortality, and India’s disease burden for diarrhoeal diseases is 2.5 to 3.5 times higher than the global average.

Predominantly sexually transmitted infections (1A60-1A9Z) 🔗

Syphilis, gonorrhoea, chlamydial infections, and trichomoniasis all sit here. These conditions carry a heavy social stigma, but classification under a neutral, standardised system helps public health programmes plan testing and treatment without judgement.

Mycobacterial diseases (1B10-1B2Z) 🔗

This is where tuberculosis lives, along with leprosy and atypical mycobacterial infections. The placement matters – India bears a quarter of the world’s TB burden, with the WHO estimating 2.8 million new TB infections annually, accounting for 26% of the global total. Standardised coding helps the National TB Elimination Programme track every notified case.

Human immunodeficiency virus disease (1C60-1C62) 🔗

HIV gets its own block in ICD-11, with sub-codes for clinical stages, opportunistic infections, and HIV-associated cancers. This is a significant improvement over earlier versions because it allows researchers to track not just whether someone has HIV, but where they are in the disease’s progression.

Arthropod-borne viral fevers (1D40-1D4Z) 🔗

Dengue, chikungunya, Japanese encephalitis, Zika, and yellow fever – all the diseases spread by mosquitoes and ticks – are grouped together. This sub-block has become increasingly important. Dengue fever, Japanese encephalitis, chikungunya, and other vector-borne illnesses remain prevalent across regions like Northeast India, where climate and geography favour mosquito breeding.

Mycoses (1F20-1F2Z) and parasitic diseases (1F40-1G2Z) 🔗

Fungal infections sit in one block, and parasitic diseases – including malaria, leishmaniasis, schistosomiasis, and intestinal worms – sit in another. India’s continuing fight against malaria, filariasis, visceral leishmaniasis and other long-standing infections depends on accurately counting cases under exactly these codes.

Why this classification matters for public health 🔗

It’s tempting to see ICD-11 as paperwork – a set of codes that doctors and hospital billers care about but nobody else does. That view misses the point. Classification is the foundation on which almost every public health decision rests.

Tracking outbreaks in real time 🔗

When a cluster of fever cases starts appearing in a district, the first question authorities ask is: what is it? If every clinic codes the suspected cases the same way – say, as 1D44 for dengue fever – the national surveillance system can detect the spike within days rather than weeks. India runs the Integrated Disease Surveillance Programme (IDSP), a World Bank-assisted project to establish a decentralized, state-based disease surveillance system that depends on exactly this kind of standardisation.

Comparing the burden across countries 🔗

A patient in Tamil Nadu coded with 1B10 for tuberculosis is, by definition, the same condition as a patient in Brazil or Botswana coded with 1B10. This makes it possible to build global disease burden estimates, plan vaccine rollouts, and direct funding to where it’s most needed. ICD-11 now includes approximately 17,000 diagnostic categories and over 130,000 clinical terms, with automated coding that reduces ambiguity.

Designing vaccination and elimination programmes 🔗

Polio surveillance in India relied on classifying every case of acute flaccid paralysis (AFP) under a strict definition. India was certified polio-free in March 2014 as part of the WHO South-East Asia Region, and outbreak-based measles-rubella surveillance was built on the same classification framework. Without a shared coding system, no elimination campaign at this scale would be possible.

Antimicrobial resistance and emerging threats 🔗

ICD-11 has built-in codes for antimicrobial resistance based on the Global Antimicrobial Resistance Surveillance System (GLASS). This is critical for a country where over-the-counter antibiotic sales remain common and resistant strains of TB and typhoid are rising. By coding resistance patterns alongside the infection itself, researchers can map which drugs are losing their punch and where.

Insurance, hospital records, and patient care 🔗

ICD codes also drive insurance reimbursements, hospital reporting, and electronic health records. As India’s Ayushman Bharat Digital Mission expands and more health data moves online, accurate ICD-11 coding will determine whether a patient’s history can be understood by any future doctor – and whether claims for treatment of conditions like dengue or TB get processed correctly.

The shift from ICD-10 to ICD-11 in India 🔗

India has used ICD-10 for cause-of-death certification and hospital records for decades. The move to ICD-11 is gradual. As of May 2024, 132 Member States and areas were at various phases of implementing ICD-11, with 14 countries already collecting or reporting data using the new codes. Countries with sophisticated existing systems usually need four to five years to transition fully.

A few changes are worth noting for anyone studying population health. Some diseases that were earlier classified by the organ system they affected – such as bacterial meningitis (previously placed under nervous system diseases) – have been moved under infectious diseases in ICD-11. This better reflects how clinicians and epidemiologists actually think about these conditions: the cause matters as much as the location.

The bigger picture: a double burden 🔗

One reason this chapter feels so relevant to population studies is that India is living through what researchers call a double burden of disease. Non-communicable conditions like diabetes and heart disease are rising sharply, but infectious diseases haven’t gone away. More than 33% of the ailing population in India still suffers from infectious diseases, with the burden falling disproportionately on rural and marginalised communities.

This means that classifying infectious diseases accurately isn’t just a technical exercise – it shapes how a country of over 1.4 billion people allocates hospital beds, plans vaccination drives, decides what gets covered under health insurance, and trains its next generation of doctors and public health workers.

What do you think? If India’s surveillance system fully transitions to ICD-11 in the next few years, which infectious diseases do you think will reveal patterns we currently miss? And how should public health planners balance the older infectious threats – TB, malaria, diarrhoea – against the rising tide of non-communicable conditions?

References

1. https://www.who.int/standards/classifications/classification-of-diseases
2. https://icd.who.int/browse/2024-01/mms/en#1435254666
3. https://icd.who.int/browse/2024-01/mms/en#979492342
4. https://dbtindia.gov.in/scientific-directorates/health-interventions-equity/infectious-diseases
5. https://en.wikipedia.org/wiki/Tuberculosis_in_India
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9. https://www.who.int/news/item/14-02-2025-who-releases-2025-update-to-the-international-classification-of-diseases-(icd-11)
10. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11358980/
11. https://www.who.int/news-room/fact-sheets/detail/icd-11
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