Raw milk can be more dangerous than you think. Researchers have only found evidence that flu viruses that end up in raw milk can infect people for about a week.

Scientists at Stanford University conducted the study, which aims to quantify the threat of exposure to influenza through contaminated milk. They found that a particular strain of the influenza A virus was still infectious after five days in refrigerated raw milk. The findings suggest that raw milk is a viable transmission route for both flu strains—especially bird flu viruses that are now actively spreading among dairy cows.

Pasteurization is the simple and brief heating of foods and liquids to kill microbes that can cause spoilage or food poisoning. Not surprisingly, extensive research shows that unpasteurized, or raw, milk products can spread harmful germs. A 2018 study, for example, found that raw milk and cheese products accounted for 96% of all reported foodborne illnesses traced back to dairy over a five-year period.

The emergence of H5N1, a strain of highly pathogenic avian influenza A (HPAI), in dairy cows this year has made raw milk even more of a gamble. People have already found H5N1 in marketed raw milk products. And some animals, especially cats living in farms, are suspected of being infected with H5N1 by drinking raw milk. But Stanford University researchers say theirs is the first research to examine the persistence of influenza in raw milk under more real-world conditions for humans.

The researchers inoculated raw milk samples with the H1N1 strain of influenza A. They used an initial dose of the virus similar to the doses found in contaminated store-brand milk products (thanks to pasteurization, even yet, no. infectious virus found in these products). Then they kept the samples chilled at a common temperature and tracked how long it took for the levels of viable virus to drop before it could infect a person. They also tested how pasteurization affected virus recovery.

Like other researchthey found that pasteurization completely removed the presence of any infectious influenza virus. But it took five days for the raw milk samples to become non-contagious.

"Overall, our study shows that influenza viruses remain infectious in raw milk, where they may pose a significant risk to human health," the researchers wrote in their paper, PUBLISHED this month's journal Letters in Environmental Science and Technology.

The study looked at a different strain of influenza A than H5N1. But other studies show that the infectiousness of both strains seems to decompose at about the same rate as milk, and that influenza A viruses generally do not differ much from each other in that respect. So H1N1 is probably a good replacement for H5N1.

As in early Decemberoutbreaks of H5N1 in dairy cows have occurred in 16 states this year, although only California and Nevada reported cases last month. In the US, 60 human cases of H5N1 have been documented, most linked to contact with infected cattle or poultry.

So far, human cases are generally mild, and the virus is not thought to have been adapted to spread easily between people.

But there are more severe cases of H5N1 reported as well. And the longer these strains are allowed to spread among cows and other mammals like us, the greater the risk that a nasty version of H5N1 will emerge and cause a widespread epidemic— a risk only amplified by the continued popularity of raw milk. About 4% of Americans are thinking about consumption raw dairy products at least once a year, with 1% of its regular consumption.

"This work highlights the potential risk of avian influenza transmission through the consumption of raw milk and the importance of milk pasteurization," said senior researcher Alexandria Boehm in a statement from the university.

Unfortunately, the same people who love raw milk products also tend to ignore any warnings about them. Selling raw milk has just skyrocketed since the emergence of H5N1 this year, even before the remember being tied to bird flu. And many followers of raw milk continue to spread misinformation about its supposed benefits over pasteurized milk, such as boosting people's immunity.