H5N1 bird flu mainly infects poultry and wild birds, but occasional human infections keep health agencies watching for pandemic risk.

Chikungunya is a mosquito-borne virus that causes fever and often severe joint pain; outbreaks can overwhelm local clinics when cases spike.

Cholera is a severe diarrheal disease spread through contaminated water or food; outbreaks follow breakdowns in clean water, sanitation, and hygiene.

COVID-19 is a respiratory disease caused by the SARS-CoV-2 virus.

Dengue is the world's most common mosquito-borne viral disease; getting infected more than once raises the risk of a severe, sometimes life-threatening case.

Ebola virus disease is a severe, often fatal illness controlled through patient isolation, supportive medical care, and close work with affected communities.

Hantaviruses spread when people breathe in dust contaminated with rodent droppings or urine, and can cause severe kidney or lung illness depending on the region.

Hepatitis A is a liver infection spread through contaminated food or water; it usually resolves on its own and is preventable with a vaccine.

Seasonal flu causes recurring outbreaks every year, while flu strains that jump from animals to people are watched closely for pandemic risk.

Lassa fever is a viral illness spread mainly by rodents in parts of West Africa, and it can also pass between people in hospitals without proper precautions.

Marburg virus causes a severe, often fatal illness with bleeding and shock; stopping outbreaks requires fast contact tracing, safe burials, and strict infection control.

Measles is one of the most contagious human viruses known; it spreads easily through the air and can cause serious complications, especially in unvaccinated children.

MERS is a severe respiratory illness linked to camels that has caused sporadic outbreaks, mostly on the Arabian Peninsula, with a notably high fatality rate.

Mpox (formerly monkeypox) is a viral illness that causes a distinctive rash; it can spread from animals to people and from person to person through close contact.

Nipah is a rare but often deadly virus that can spread from bats or pigs to people, mostly in South and Southeast Asia.

Norovirus is a highly contagious stomach bug that causes sudden vomiting and diarrhea, and it spreads easily in close-contact settings like cruise ships and care homes.

Oropouche is a virus spread mainly by tiny biting midges that has caused growing outbreaks in the Americas, usually causing dengue-like fever.

Polio is a viral disease that can cause permanent paralysis; decades of vaccination have brought it to the edge of global eradication, but pockets of transmission remain.

Rabies is a virus that attacks the brain and is almost always fatal once symptoms begin, but it is entirely preventable with prompt treatment after a bite.

RSV is a common respiratory virus that causes cold-like illness in most people but can be dangerous for infants and older adults.

West Nile virus is a mosquito-borne infection that is usually mild but can occasionally cause serious brain inflammation, particularly in older adults.

Yellow fever is a mosquito-borne viral illness that can cause serious liver disease and bleeding; a single vaccine dose offers long-lasting protection.

Zika is a virus spread mainly by mosquito bites that causes outbreaks in tropical and subtropical regions, with special risks for pregnant women and their babies.