Bird flu: What are the human symptoms and how does it spread?

Earlier this month, Colorado Gov. Jared Polis declared a disaster emergency not for the wildfires common to the Centennial State this time of year, but for a bird flu outbreak at a commercial poultry facility in Weld County, north of Denver. Now, several workers at the egg farm who had been culling poultry in response to the spread of avian influenza have tested positive for the disease.

Five people have been infected, according to the Colorado Department of Public Health and Environment. As of Sunday, the Centers for Disease Control and Prevention (CDC) had confirmed four of those cases. None of the workers was hospitalized, Colorado officials said, noting their mild symptoms included conjunctivitis, or pink eye, and symptoms consistent with respiratory infection.

Colorado was already ground zero for animal-to-human spread of H5N1, the current strain of bird flu circulating the globe. In 2022, a Colorado farm worker culling poultry tested positive. Until this month, CDC records show, no other poultry-to-human transmissions had been reported. However, since April 1, 2024, four workers on U.S. dairy farms have tested positive for bird flu after interacting with cattle: two in Michigan, one in Texas, and, most recently, one in Colorado.

If the presumptive positive test from Colorado’s latest poultry outbreak is confirmed, 10 people in the U.S. will have contracted bird flu from cattle or poultry since 2022. The CDC maintains that the general public’s risk of contracting H5N1 is low. People who work closely with infected birds and other animals, though, remain at greater risk.

Infected wild birds can spread the virus to poultry and other animals through feces, mucus, and saliva. Avian flu has so far impacted more than 99 million poultry in 48 states and more than 150 dairy cattle in 12 states. Modern industrial farming has crafted ideal conditions for disease spread, according to Edwin Michael, PhD, an epidemiologist at the University of South Florida College of Public Health.

“Look at the density of animals in those farms—those are not natural settings,” Michael previously told Fortune. “So as soon as you get [an infection], these things will spread very quickly among farm animals.”

Michael added, “We have to shed a light on how farming is done. That’s the trade-off, you want cheap meat and all the rest of it, but then you farm animals in this way and you’re opening the door up for other things.”

How does bird flu spread to humans?

The general public has a low risk of catching bird flu not only because they’re unlikely to come into direct contact with infected birds but also because the CDC hasn’t documented any person-to-person transmission.

Testing by the Food and Drug Administration and the U.S. Department of Agriculture has shown the nation’s commercial milk supply is safe, as pasteurization inactivates H5N1. Eggs and poultry are also safe to eat, so long as they are cooked to an internal temperature of 165 degrees, the CDC says.

In April, the World Health Organization seconded the CDC’s assessment that the public was at minimal risk, citing a lack of evidence that the virus had acquired mutations that would facilitate its transmission among humans. The agency stressed that sporadic bird flu infections in mammals and humans is par for the course, and that further human cases wouldn’t be unexpected. This guidance has not yet been updated following the latest string of Colorado infections.

Preventing widespread infection among farm workers is key to limiting opportunities for viral mutation, says Michael Osterholm, PhD, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.

“There’s no evidence yet at this point that this is an imminent risk to humans,” Osterholm told Fortune in May. “We’ve not seen it cross over to humans in a way that would support that it’s going to be the next pandemic virus. On the other hand, a reassortant event, or continued mutations, could occur tomorrow.”

What are symptoms of bird flu in humans?

Humans infected with bird flu display a wide range of symptoms, from pneumonia requiring hospitalization to no symptoms at all. The CDC lists these other possible signs of infection: