The man, described by Colorado health officials as being younger than 40 years old, experienced only fatigue; he is now in isolation and being treated with the flu antiviral oseltamivir.
H5 viruses have long struck fear among scientists who study bird flu because of the economic damage they can wreak and their propensity to occasionally infect people.
But when the first pandemic in decades finally occurred in 2009, it was caused by a mild swine flu virus, H1N1.
The test may have picked up virus that was “surface contaminant” — presumably virus picked up by a nose swab — that was present because of the man’s involvement in culling infected poultry.
This is only the second human infection caused by this clade of H5N1 viruses; the first was a man in the United Kingdom who was infected last December.