A highly contagious strain of avian flu that has likely killed hundreds of birds and spread across more than two dozen states has been detected in a human for the first time in the U.S., officials said Thursday.

The man was working on a commercial farm in Colorado and was involved in culling poultry suspected to be infected when he was directly exposed to the H5N1 flu, the state’s health department said in a release.

The man, described as younger than 40, has reported only one symptom — fatigue — and was taking the antiviral drug Tamiflu, the department said.

A positive test administered this week by the state health department, which said it has been monitoring people exposed to poultry and wild birds, was confirmed Wednesday by the Centers for Disease Control and Prevention, the agency said.

Health officials still don't know with certainty whether the man was actually infected with the virus or if the virus had only contaminated the surface of his nose at the time the test was taken, Colorado state epidemiologist Rachel Herlihy told NBC News on Friday.

"With all that said, we are certainly being cautious here and treating this like it could be an infection," she said.

The state health department described the man as a prison inmate who was working at a Montrose County farm as part of a pre-release employment program.

The farm had recently experienced an outbreak of the H5N1 bird flu, and he and other incarcerated individuals had been tasked with helping to euthanize the flock, Herlihy said.

They were given personal protective equipment, though it's unclear whether all of it was being properly used, she said.

The other incarcerated people are being monitored but have tested negative for the virus. The man, who tested positive, has since tested negative but has been asked to remain in isolation until Saturday, she said.

The state health agency and the CDC said Thursday that the risk the virus poses to people still remains low.

Although public health officials have worried that a mutation could present a threat to humans, only one other human case has been detected worldwide — a person who raised birds in the United Kingdom tested positive for the virus in December. The person was asymptomatic, the CDC said.

What is H5 bird flu?

There are three types of H5 viruses that have infected birds in the U.S.: H5N1, H5N2, and H5N8.

The Colorado man tested positive for an H5 bird flu virus, Herlihy said.

The H refers to hemagglutinin, one of two proteins that sit on the surface of influenza viruses and allow the viruses to enter cells.

Herlihy said work is underway to identify the second protein, the neuraminidase subtype, or N, of the virus present in the man.

It's assumed to be N1, Herlihy said, because there aren't other H5 viruses besides H5N1 circulating in the U.S. right now.

Other kinds of influenza, namely H1 and H3, are the ones that usually cause outbreaks in humans.

Most H5 viruses identified worldwide are known to infect wild birds and poultry, but can sometimes infect humans, said Dr. Buddy Creech, an infectious disease expert at the Vanderbilt University Medical Center in Nashville, Tennessee.

Earlier versions of the virus infected roughly 880 people beginning in 2003, the agency said. Although the virus rarely infects people, it can be severe when it does: According to the CDC, its mortality rate is 60 percent.

The H5N1 virus now circulating has been found in commercial and backyard birds in 29 states and in wild birds in 34 states, the CDC said.

An outbreak at a lake outside Chicago is believed to have killed more than 200 birds, and at least three bald eagles died from the virus in Georgia. Millions of chickens and turkeys have been killed to prevent the virus's spread, prompting a surge in poultry prices.