HARRISBURG (TNS) — The official number of people that have contracted bird flu in the past year, 68 to be exact, has been relatively small.

A new study from the Centers for Disease Control and Prevention (CDC) suggests that the true number of infected people is actually higher.

The study, published on Thursday, said that bird flu had spread from animals to some veterinarians, even though the vets could not recall having any symptoms.

“This means that people are being infected, likely due to their occupational exposures, and not developing signs of illness and therefore not seeking medical care,” Dr. Gregory Gray, an infectious disease researcher at the University of Texas Medical Branch in Galveston, said to PBS.

The study took blood samples of 150 veterinarians from 46 states that worked with cows and cattle. Of those 150, three had antibodies to H5N1, or bird flu, infection, although none of the three said they had worked with any infected cattle, although one did work with infected poultry.

“I think the bottom line here is that there are vets who may have been infected in states that hadn’t reported outbreaks, which is bad,” Dr. Jennifer Nuzzo, director of the Pandemic Center at Brown University said to CNN. “It speaks to the need to improve our surveillance so that we can better detect when outbreaks are occurring on farm so we can protect people.”

The good news is that recent studies suggest people who become infected mount antibody responses and might develop a natural immunity. Although the virus could still mutate and start making sicker at some point.

“No one’s really questioning that the virus has been moving around the country more than has been reported,” said Keith Poulsen, director of the Wisconsin Veterinary Diagnostic Laboratory, said to PBS.