A vet at a New York City animal shelter where cats were infected with a strain of bird flu has also been infected, health officials said Thursday.
The vet only had mild illness from the virus, called H7N2, the New York City Department of Health said.
"The illness was mild, short-lived and has resolved," the department said in a statement.
"More than 160 Animal Care Centers of NYC employees and volunteers, including several people who had similar exposure to sick cats, were screened by the Health Department and not found to have infection with the H7N2 virus," the department said.
Related: Bird Flu Infects Cats at NYC Shelter
"Additionally, the Health Department contacted more than 80 percent of the people who adopted cats from the Manhattan shelter, and none is suspected of having H7N2," the department added.
It's unusual for bird flu to affect cats.
"Since last week, more than 100 cats have tested positive for H7N2 across all NYC shelters," the department said. "This was expected because the virus is highly contagious among cats and cats are sometimes moved between shelters. All of the newly infected cats are experiencing mild illness and have been separated from other animals in the shelters. They are expected to recover."
Vets, doctors and other scientists keep an eye on bird flu because it can and does spread to people and has the potential to cause epidemics. So far, H7N2 hasn't. It has only ever infected a few people.
Related: Keep an Eye Out for H5N2, CDC Says
Still, influenza worries doctors.
"As a general rule, influenza is what concerns us most," Centers for Disease Control and Prevention director Dr. Thomas Frieden said.
"We are always concerned about the possibility of re-assortment between animal flu and human flu so, this is something that we have been watching very closely."
Flu viruses mutate constantly and swap big pieces with one another. New pandemics of influenza come when a "new" strain emerges from this mixing process, called re-assortment. Especially worrying is when a strain that easily infects people picks up bits and pieces from animal strains, creating an easily transmitted flu strain that could also be deadly.
Related: H7N9 Flu Spreads Like Ordinary Flu
That doesn't appear to have happened with H7N2 among the cats. The vet who was infected had been testing the sick cats, so had prolonged, close contact with them.
"But any time we have a new strain of flu spreading in animals and in humans we are very much on the alert," Frieden said.
Ordinary influenza is bad enough, killing anywhere from 4,000 to 50,000 Americans a year. New pandemic strains can kill many times that.
People can and do catch flu from animals — and vice-versa. Pigs commonly both catch flu from people and transmit it. The ongoing occasional human cases of H5N1 bird flu and H7N2 bird flu are traced to poultry markets.