Health and Medicine
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As bird flu continues to cause problems around the world, a patient in the US has tested positive for H5N5, a subtype of avian influenza never before seen in humans.
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Officials noted that the person kept domestic poultry at home that had been exposed to wild birds. This is likely to be the source of the infection, health authorities said, although their investigations are ongoing.
Avian influenza, commonly called bird flu, is a viral disease caused by the influenza A virus. As its name suggests, it primarily affects birds, which – given their long migrations and large-scale farming – can potentially spread the virus quickly if a virulent strain emerges. The virus can also occasionally jump to mammals, including humans, which is exactly what has been happening with the persistent bird flu outbreak that started in 2020 and ramped up in 2024.
As of November 17, 2025, the US Centers for Disease Control and Prevention (CDC) has identified 71 cases of bird flu in the country, including one reported death. The majority of these cases have been mild illnesses in people who work on dairy and poultry farms.
The highly pathogenic influenza A virus subtype H5N1 was responsible for these infections. This latest case in Washington State is caused by a slightly different subtype, H5N5, which had only been detected in non-human animals before.
While it marks a significant moment in the ongoing bird flu outbreak, the CDC and the Washington State Department of Health say the risk to the public remains low.
The difference between H5N1 and H5N5 comes down to a protein in the virus’s surface. It’s unclear how this might affect the virus’s virulence or severity. However, for farmers and doctors concerned about H5N5, the virus is expected to behave similarly to H5N1, producing comparable symptoms in livestock and humans.
"These viruses behave similarly. My gut instinct is to consider it the same as H5N1 from a human health perspective," Richard Webby, a prominent flu researcher at St. Jude Children's Research Hospital in Memphis, Tennessee, told the Associated Press.
“Think along the lines of different brands of car tires. They both do the same job, it’s just each is better tuned for specific conditions, which we don’t fully understand,” Webby explained.
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