Washington state has reported the first suspected human case of H5N1 bird flu in the US since February.
The patient, an older adult with underlying conditions, remains in hospital as investigations continue.
Officials have not yet identified the source of infection and are tracing potential contacts.
Experts warn the virus still carries “pandemic potential”, though human transmission remains difficult.
A resident of Washington state in the United States has tested positive for avian influenza, or bird flu, in what could be the first human case reported in the country since February, health officials have said.
Preliminary findings indicate the infection involves H5N1, a highly pathogenic avian influenza (HPAI) strain. The case is now being investigated by the Washington State Department of Health and the US Centers for Disease Control and Prevention (CDC).
According to CDC data, 70 human H5N1 infections have been identified in the US since 2024 till July 2025, mostly among poultry and dairy farm workers exposed during an outbreak involving the virus spreading in dairy cattle.
The latest patient has been identified as an older adult with underlying health conditions from Grays Harbor County. The individual was admitted to hospital in early November with fever, confusion and respiratory distress, and is continuing to receive treatment.
“To ensure that human-to-human spread is not occurring, public health officials are contacting anyone who has been in close contact with the patient to monitor for symptoms and provide testing and treatment as needed,” Washington health officials said in a statement.
Authorities have not yet determined how the person became infected, including whether exposure to wild or domestic birds was involved.
Richard Webby, an avian influenza specialist, has warned that although the virus still faces “biological hurdles” before it can spread easily among people, its future trajectory is uncertain. “It’s not an easy leap for this virus to switch from being a duck virus to a human virus,” he said, adding that it still carries “pandemic potential”.
Speaking to news website CNN, he added: “It’s going to take a little bit of the biologic stars aligning for that to happen. We could argue exactly how likely that is, but no one actually knows. Only time will tell us, unfortunately.”
A study published in the journal Nature in October suggested that the H9N2 avian influenza virus has become better adapted to infect humans, although no evidence of human-to-human transmission has been reported. The authors warned that this lesser-known strain, which typically causes mild disease in birds, could pose a human pandemic risk.
The Washington health department in a statement noted that the risk of avian influenza typically rises in autumn and winter, as migratory birds can spread the virus to domestic poultry, including commercial farms and backyard flocks.
In the past 30 days, 76 outbreaks of H5N1 have been confirmed across 38 commercial and backyard flocks in the US, affecting around 1.66 million birds, according to the Animal and Plant Health Inspection Service (APHIS). With no evidence of person-to-person spread in the current case or elsewhere in the country, the CDC continues to assess the overall risk to the US public as low.