Bird flu: Why health experts are so worried about raw milk.

(NEXSTAR) – As the avian influenza virus, type A H5N1, continues to spread to an unknown number of dairy cows, health experts are informing the public milk and other dairy products are still safe to consume – with one major exception.

When a cow is infected with bird flu, high amounts of the virus can be detected in its milk. However, the pasteurization process kills off or inactivates the virus, the Food and Drug Administration says.

Raw milk, on the other hand, is not pasteurized. "Raw milk can be contaminated with harmful germs that can make you very sick," the CDC says on its website.

It’s not yet known whether the live virus can be transmitted to people who consume milk that hasn't been heat-treated through pasteurization, but there are documented cases of humans becoming sick from eating undercooked, tainted poultry.

"Raw milk and raw milk products may not undergo the same processes to inactivate the virus, and so I have very large concerns about the safety of raw milk," said Meghan Davis, an associate professor at the Johns Hopkins Bloomberg School of Public Health.

There's also evidence that animals can get sick from consuming raw milk, Davis said. "We know on dairy farms that cats on the farm that have been consuming the raw milk have been getting very sick with neurologic, as well as respiratory, disease and high mortality."

Aside from bird flu concerns, raw milk can also contain salmonella, E. coli and listeria, the FDA says.

Yet raw milk sales have been on the rise, according to the market research firm NielsenIQ. Between mid-March (when the bird flu virus was confirmed in U.S. cattle for the first time) and mid-May, weekly sales of raw cow’s milk have ticked up 21% to as much as 65% compared with the same periods a year ago.

Mark McAfee, owner of Raw Farm USA in Fresno, California, said he can’t keep his unpasteurized products in stock.

“People are seeking raw milk like crazy,” he said, noting that no bird flu has been detected in his herds or in California. “Anything that the FDA tells our customers to do, they do the opposite.”

The surge surprises Donald Schaffner, a Rutgers University food science professor who called the trend “absolutely stunning.”

“Food safety experts like me are just simply left shaking their heads,” he said.

Before milk standards were adopted in 1924, about 25% of foodborne illnesses in the U.S. were related to dairy consumption, said Alex O’Brien, safety and quality coordinator for the Center for Dairy Research. Now, dairy products account for about 1% of such illnesses, he said.

“I liken drinking raw milk to playing Russian roulette,” O’Brien said. The more times people consume it, the greater the chance they’ll get sick, he added.