PESCADERO, CALIFORNIA – DECEMBER 20: A view of chickens and a rooster at a farm as California declares state of emergency to prevent new public health crisis on Bird flu in Pescadero, California, United States on December 20, 2024. (Photo by Tayfun Coskun/Anadolu via Getty Images)
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New details have emerged about a San Francisco child infected with H5N1 bird flu late last year, one in a string of U.S. cases since 2023. Unlike most of those, this infection had no clear source, raising questions about where the virus is persisting, particularly in urban environments, and how human exposures occur.
The case, described by the San Francisco Department of Public Health and CDC in the Morbidity and Mortality Weekly Report on September 4, 2025, involved a school-aged child who fell ill in December 2024.
Their illness looked like an ordinary flu: fever, abdominal pain, muscle aches, conjunctivitis. Symptoms lasted about a week, and by the time H5N1 was flagged through enhanced surveillance—weekly batch testing of samples to identify virus subtypes—the child was already better.
Authorities traced 67 close contacts—including household members, classmates, and health care providers—but found no evidence that the child had transmitted to anyone else. Although only a small fraction of contacts were tested, all samples were negative. That caveat leaves open the possibility that very mild or asymptomatic infections could have been missed, though no evidence points to onward spread.
Contact tracing of a child with highly pathogenic bird flu in California, December 2024. Original image published at https://www.cdc.gov/mmwr/volumes/74/wr/mm7433a2.htm.
Farrell A. Tobolowsky et al.
What makes this case stand out is the missing link. The child hadn’t traveled, hadn’t visited farms, and hadn’t been around poultry or wild birds. The family had purchased poultry from a live bird market, a major risk factor in other outbreaks, but it was cooked and eaten well before the illness began and the child had not been to the market.
Genetic sequencing showed the virus belonged to a group of viruses known as clade 2.3.4.4b, genotype B3.13, which is the same lineage driving the California dairy outbreak and showing up in cats and wild birds. Crucially, the genetic sequence lacked mutations thought to enable efficient human-to-human transmission. Still, the fact that the child was infected without a clear exposure highlights how much we don’t yet understand about the environmental reservoirs or transmission pathways of this virus in urban settings.
This case is one of 70 that have been confirmed in the U.S. since 2023. Most have been linked to dairy cattle or poultry, but in three—including this one—investigators were unable to identify a source of exposure.
PETALUMA, CALIFORNIA – APRIL 26: Cows graze in a field at a dairy farm on April 26, 2024 in Petaluma, California. The U.S. Department of Agriculture is ordering dairy producers to test cows that produce milk for infections from highly pathogenic avian influenza (HPAI H5N1) before the animals are transported to a different state following the discovery of the virus in samples of pasteurized milk taken by the Food and Drug Administration. (Photo by Justin Sullivan/Getty Images)
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So far, American cases have mostly been mild, though there has been one death. That contrasts sharply with the global history of H5N1, where many infections in Asia, the Middle East, and Africa have been fatal. Virologists caution that although American strains mostly lack key mutations for sustained person-to-person transmission, adaptation to mammals has occurred elsewhere, including in marine mammals including sea lions in South America and in mink, foxes, and raccoon dogs on fur farms in Europe.
The report is a reminder of both the potential and the limits of current surveillance. Batch testing caught the infection, but only after the illness was over—too late for isolation or antiviral treatment, and too late to understand exposures in real time.
While it is fortunate that the child’s case was mild, the questions that remain about how the child became infected in the first place underscore how much there is still to learn about the epidemiology of bird flu in North American and the importance of surveillance, rapid diagnostic capacity, and protection for people in contact with potentially infected animals.
It seems to me that the real challenge for public health is how to accurately assess risk to the public—the CDC currently rates it as “low”—when there is still much that remains unknown.