How many viruses infect humans?

The commonly cited answer is that about 270 viruses are known to infect humans. That figure comes from a 2022 review, not a continuously updated inventory. Scientists keep finding viruses and revising how species are classified.

A broader 2022 atlas found 1,131 virus species in human samples. Not all 1,131 are proven to reproduce in human cells or cause disease. Each study sets its own threshold for what counts as evidence.

How many viruses infect humans?

The best concise estimate is about 270 known human-infecting viruses, based on a 2022 review.^[1]

The newest detailed census covers RNA viruses only. It lists 239 species with published evidence of natural human infection as of December 2024, up from 214 in 2018. Thirteen additions were newly discovered species. The others reflect taxonomic changes and revised inclusion rules.^[2]

That census sets a high evidence bar. A virus must be recognized as a species by the International Committee on Taxonomy of Viruses and supported by a peer-reviewed report of natural human infection. Detection in cultured human cells or deliberate experimental infection does not qualify. A virus does not have to cause symptoms to count.^[2]

Most of these RNA viruses also infect other animals. Of the 239 species, 147 are strictly zoonotic, meaning known human cases come from non-human hosts, and only 24 are currently known to infect humans alone. The remaining species include viruses that can spread in limited outbreaks or circulate endemically in people.^[2]

Why do estimates range from 219 to 1,131?

Published totals range from 219 to 1,131 because the studies count different kinds of evidence, use different taxonomies, and cover different dates.

A 2012 literature survey counted 219 recognized virus species with evidence of natural human infection. It found species from 23 families and noted that new human viruses were being identified at a rate of three or four per year.^[3]

The 2022 review put the concise estimate at about 270 known human-infecting viruses.^[1] A separate 2022 Human Virus Database study used a broader rule, collecting animal-virus sequences isolated or detected in human tissues, fluids, and excreta. That atlas contained 1,131 species from 54 families, including 433 DNA viruses, 656 RNA viruses, and 42 reverse-transcribing viruses.^[4]

The chart compares published totals under different inclusion rules, not growth over time.^[1][3][4]

The atlas identifies viral signals that may deserve further study. Its total is not a count of 1,131 confirmed human pathogens. Finding viral sequence in a blood, stool, or tissue sample does not prove that the virus productively infects human cells.

These species totals are also different from the number of virus particles in one person. The human virome includes viruses that infect our bacteria, while inherited viral DNA includes ancient retroviral sequences that are now part of the human genome.

How many virus families infect humans?

Twenty-six virus families are commonly listed as causes of human disease.^[5]

The 26-family figure comes from a clinical grouping and changes with taxonomy. The 2012 natural-infection survey counted 23 families across all genome types.^[3] The 2026 RNA-virus catalogue also counts 23 families, but only for its 239 human-infective RNA virus species.^[2]

The broader Human Virus Database found 54 families because it included virus species detected in human samples under more inclusive rules.^[4] Family names and boundaries also change when taxonomists split, merge, or reassign groups. The evidence rule and genome scope belong beside any family count.

How many types of flu virus affect humans?

Three types of influenza virus affect humans: influenza A, B, and C.^[6]

Four influenza types are recognized in total: A, B, C, and D. Types A and B cause seasonal epidemics in people. Type C usually causes mild illness and is not associated with human epidemics. Type D mainly infects cattle and is not known to cause illness in people.^[6][7]

Influenza A is divided into subtypes by two surface proteins. Scientists recognize 18 hemagglutinin, or H, subtypes and 11 neuraminidase, or N, subtypes. Eight H subtypes and six N subtypes have been detected in humans, while more than 130 H-N combinations have been identified in nature, mostly in wild birds.^[6][7]

Only A(H1N1) and A(H3N2) routinely circulate in people. Influenza B is divided into lineages rather than subtypes. B/Victoria still circulates, while B/Yamagata has not been detected since March 2020.^[7]

CDC groups flu viruses further into clades and subclades by comparing gene sequences. Those relationships can be examined with a multiple sequence alignment and a phylogenetic tree.

Current U.S. flu vaccines are trivalent. They target an A(H1N1) virus, an A(H3N2) virus, and a B/Victoria-lineage virus.^[8]

Sources▼

1. Disease-causing human viruses: novelty and legacy Trends in Microbiology · 2022. https://pubmed.ncbi.nlm.nih.gov/35902319/
2. A complete catalogue of human-infective RNA viruses Scientific Data · 2026. https://www.nature.com/articles/s41597-026-07281-5
3. Human viruses: discovery and emergence Philosophical Transactions of the Royal Society B · 2012. https://pmc.ncbi.nlm.nih.gov/articles/PMC3427559/
4. An atlas of human viruses provides new insights into diversity and tissue tropism of human viruses Bioinformatics · 2022. https://academic.oup.com/bioinformatics/article/38/11/3087/6569817
5. Classification of Human Viruses Principles and Practice of Pediatric Infectious Diseases · 2018. https://pmc.ncbi.nlm.nih.gov/articles/PMC7151951/
6. Chapter 12: Influenza CDC Pink Book · July 30, 2026. https://www.cdc.gov/pinkbook/hcp/table-of-contents/chapter-12-influenza.html
7. Types of Influenza Viruses Centers for Disease Control and Prevention · July 30, 2026. https://www.cdc.gov/flu/about/viruses-types.html
8. Selecting Viruses for the Seasonal Influenza Vaccine Centers for Disease Control and Prevention · July 30, 2026. https://www.cdc.gov/flu/vaccine-process/vaccine-selection.html