Open Outbreak

Avian influenza A(H5N1)

Also known as: H5N1, bird flu, highly pathogenic avian influenza, HPAI, avian influenza A(H5N1) virus

Avian influenza A(H5N1) is a flu virus that mainly infects birds, but it can sometimes infect people. Most human infections reported to WHO have followed close contact with sick or dead birds, other infected animals, or contaminated environments. WHO says these infections are usually sporadic, meaning they happen one by one rather than as ongoing chains of spread between people. Illness in humans can range from mild symptoms, including conjunctivitis, to severe pneumonia and death. As of 2025-07-01, WHO had recorded 986 human cases and 473 deaths reported from 25 countries since 2003, though this figure reflects detected and reported cases, not all infections. Antiviral medicines such as oseltamivir and zanamivir may help, especially when given early. Vaccines exist as preparedness tools in some settings, but there is no widely used routine public vaccine program for H5N1. WHO’s current assessment in the 2025 Cambodia notice is that the risk to the general population is low, while risk is higher for people with occupational exposure to infected animals.

Key facts

TransmissionMostly from close contact with infected live or dead birds, some infected mammals, or contaminated environments. Sustained human-to-human transmission has not been documented by WHO.
IncubationOften a few days after exposure; one 2024 WHO report described bird exposure in the third week of 2024-02 and symptom onset on 2024-03-11, showing that incubation can be longer than seasonal flu. Exact range is not clearly established in the sources used here.
Case fatalityWHO reported 473 deaths among 986 human cases globally from 2003 to 2025-07-01 (about 48%). In Cambodia, WHO reported 49 deaths among 83 cases from 2003 to 2025-07-01 (59%), and 12 deaths among 27 cases since re-emergence in 2023 (44%).
Reproduction numberunknown; WHO says sustained human-to-human transmission has not been documented in the sources used here.
VaccineCandidate H5 vaccines exist for preparedness, but the sources used here do not describe a broadly deployed routine public vaccine program.
TreatmentSupportive care plus antivirals. WHO notes neuraminidase inhibitors such as oseltamivir and zanamivir may shorten viral replication and improve outcomes in some cases, especially when started within 48 hours of symptom onset.
ReservoirWild birds and poultry are the main reservoir; WHO also notes circulation in some mammals.
Endemic regionsNot confined to one human endemic zone. The virus circulates in wild birds, poultry and some mammals around the world, with human cases reported to WHO from 25 countries since 2003.

Transmission

H5N1 is mainly an animal infection, not a human one. In the WHO reports returned here, almost all human cases were linked to close contact with infected live or dead birds, other infected mammals, or contaminated environments. Examples include backyard poultry exposure in Cambodia and exposure to dairy cattle in the United States. WHO repeatedly says the virus does not easily infect humans and that sustained human-to-human transmission has not been documented. That matters because a disease becomes much harder to control when it spreads efficiently from person to person. Some infections found through surveillance have been asymptomatic, meaning the person tested positive without feeling ill. For a general reader, the main practical message is that risk is highest around infected animals, carcasses, droppings, raw milk or other contaminated farm settings, not in ordinary day-to-day contact with the public.

Symptoms and severity

Human H5N1 infection can look very different from one person to another. WHO says illness may range from mild upper respiratory symptoms to severe, life-threatening disease. Reported features include conjunctivitis, cough, shortness of breath, gastrointestinal symptoms, encephalitis and encephalopathy. Severe pneumonia is a major concern. The overall severity in detected human cases has been high: WHO’s Cambodia update reported 986 human cases and 473 deaths worldwide from 2003 to 2025-07-01, a case fatality ratio of about 48%. That figure probably reflects the more serious cases that come to medical attention, so it should not be read as the risk after every exposure. Still, it shows why health authorities take each human case seriously. In Cambodia, WHO reported 83 cases and 49 deaths since 2003, and since re-emergence in 2023 it reported 27 cases with 12 deaths. Children have made up many recent Cambodian cases.

Treatment and vaccines

Treatment is mainly supportive care plus antiviral medicine. WHO states that neuraminidase inhibitors, especially oseltamivir and zanamivir, have been shown in some cases to shorten the duration of viral replication and improve outcomes, and that treatment should start within 48 hours of symptom onset when possible. That does not guarantee recovery, but early treatment is one of the main tools available. People with severe disease may also need oxygen, intensive care and treatment for complications such as pneumonia. On vaccines, the sources used here support a cautious summary: H5 vaccines and vaccine candidates exist for pandemic preparedness, but the returned WHO outbreak notices do not describe a broadly deployed routine vaccine program for the public. In plain terms, H5N1 is not like seasonal flu, where a widely available yearly vaccine is standard in many countries. Control still depends heavily on animal outbreak control, surveillance, protective equipment for exposed workers and rapid testing of suspected human cases.

Where it occurs

H5N1 is best understood as a global animal health problem with occasional human spillover. WHO’s 2025 Cambodia notice says the virus is circulating in wild birds, poultry and some mammals around the world. Human cases reported to WHO from 2003 to 2025-07-01 came from 25 countries. Recent WHO notices in the tool results show human infections or detections linked to Cambodia, Viet Nam, the United States, Australia, Mexico, the United Kingdom and earlier events in Spain and Chile. The settings vary. In Cambodia, many cases were linked to backyard poultry. In the United States, WHO described exposure to infected dairy cattle, which was notable because it suggested mammal-to-human transmission. Australia’s 2024 case was likely exposed during travel in India. So the map is not a single endemic human belt. Instead, risk tends to follow infected animal populations, trade, migration of wild birds and work that brings people into close contact with affected animals or contaminated farm environments.

Recent history

On this site’s archive, H5N1 has 71 archived events since 1996, showing that it is a long-running public health issue rather than a one-off scare. The recent pattern in WHO notices is still mostly sporadic human infection, but with changing animal hosts and new countries affected. Cambodia reported repeated human cases in 2023, 2024 and 2025; WHO called the seven Cambodian cases reported in 2025-06 an unusual monthly increase. In 2024, WHO reported a human case in Viet Nam, the first human H5N1 case reported by Australia, and a United States case linked to dairy cattle. In 2025, WHO reported Mexico’s first confirmed human infection with an A(H5N1) virus. Despite these developments, WHO’s assessments in the recent notices still say the available epidemiological and virological evidence does not show sustained human-to-human transmission. The disease therefore remains a serious zoonotic threat with pandemic potential, but not one that is currently spreading widely between people according to the sources used here.

What to watch

The biggest warning sign would be clear evidence that H5N1 is spreading efficiently from person to person. Right now, WHO says sustained human-to-human transmission has not been documented. Other signals that would raise concern are clusters without a clear animal exposure, spread into new regions or animal populations, and changes in the virus that make infection in mammals more common. The 2024 United States report matters because it involved exposure to dairy cattle, not birds, and the 2025 Mexico report shows another country with a confirmed human A(H5N1) infection. Drug resistance would also matter, because current treatment options are limited and depend heavily on antivirals such as oseltamivir. Finally, vaccine supply and access would become important quickly if human transmission changed. For now, the key public health watch points are animal outbreaks, exposed workers becoming ill, unusual family or healthcare clusters, and any WHO change in risk assessment from low for the general population to something higher.

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