A Colorado prison inmate has become the first person in the U.S. to test positive for bird flu in a recent outbreak that has led to the deaths of millions of chickens and turkeys, but federal officials say they still see little threat to the general public.

The U.S. Centers for Disease Control and Prevention said Thursday evening that the infected man had been in a prerelease program and was helping to remove chickens from an infected farm. The man, who was under age 40, reported fatigue for a few days but has recovered, state health and CDC officials said in a statement.

The man was isolated and is being treated with an antiviral drug. Other people involved in the bird removal operation in Colorado have tested negative, but they are being retested out of an abundance of caution.

The man was part of a crew of inmates nearing release who had been working at the farm before a case of bird flu was confirmed there on April 19, said Lisa Wiley, a spokeswoman for the Colorado Department of Corrections. When bird flu was detected at the farm in Montrose County, the inmates were asked to help kill and remove the birds.

Agriculture officials have reported an outbreak at one Montrose County farm with 58,000 broiler breeder chickens.

Despite the infection, the CDC considers the threat to the general public to be low because spread of the virus to people requires close contact with an infected bird.

Signals that could raise the public health risk might include multiple reports of virus infections in people from exposure to birds, or identification of spread from one person to another. The CDC also is monitoring for genetic changes to the H5N1 bird flu virus that is currently circulating. Any genetic changes could indicate the virus is adapting to spread more readily from birds to people or other mammals.

Many different bird flu viruses have infected humans worldwide since at least the 1990s, but health officials still say human infection is uncommon.

In 2002, H7N2 caused pinkeye and mild respiratory symptoms in people in the U.K. and the U.S. Four U.S. infections have been identified since 2002; two were transmitted from cats to humans in 2016.

More than 1,500 people in China have been infected with the H7N9 strain, largely in outbreaks between 2013 and 2017. This version caused serious infections among people and 40% of those who were hospitalized died.

A different variant of H5N1 has also circulated since 1997, infecting more than 880 people, and it had a 50% case fatality rate.

The current H5N1 variant has been spreading among backyard and commercial chicken and turkey flocks in the U.S. since late February. Viruses have been found in U.S. commercial and backyard birds in 29 states and in wild birds in 34 states. More than 35 million chickens and turkeys have been killed and removed to avoid spread, the U.S. Department of Agriculture reported.

The CDC said it has tracked the health of more than 2,500 people who have been exposed to H5N1-infected birds but that the inmate’s illness was the only confirmed case to date.

The agency said it was possible the man only had the virus present in his nose and that his body was not infected. Colorado public health officials say repeat testing on the man was negative for influenza. A nasal swab positive test result meets the agency’s criteria for considering it an infection.

“The appropriate public health response at this time is to assume this is an infection and take actions to contain and treat,” the CDC statement said.

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