Most people think aging cheese makes it safer to eat. But new research from Cornell University has revealed something that could change how we see raw-milk products forever. Scientists found that the bird flu virus, known as H5N1, can survive for up to 120 days in raw-milk cheese.
“In this study, we were specifically looking at the stability or persistence of highly pathogenic avian influenza H5N1 in raw milk cheese products,” said Dr. Diego Diel, professor of virology at Cornell University.
This discovery challenges long-held safety rules and highlights how infectious viruses can linger in foods many assume are harmless. The study was published in Nature Medicine and led by a team of food scientists and veterinarians from Cornell and the U.S. Food and Drug Administration.
Fast Facts
- Study Source: Cornell University and U.S. FDA research published in Nature Medicine (2025).
- Key Finding: The H5N1 bird flu virus can survive up to 120 days in raw-milk cheese.
- Risk Factor: Standard 60-day aging rules don’t eliminate the virus in unpasteurized cheese.
- Safe Practice: Pasteurization or strong acidification destroys the virus before cheesemaking.
- Public Impact: Raises global food safety concerns for raw dairy consumers and producers.
The Study That Changed What We Know About Raw Milk
Image credit: Cornell University College of Veterinary Medicine / Nature Medicine (2025). Panel (a) created with BioRender.
The research began after bird flu spread from wild birds to dairy cows in the United States. Some infected cows started producing milk with high levels of the virus. This raised a serious question. Could unpasteurized milk or cheese made from it infect humans?
To find out, the scientists made cheese from milk containing the H5N1 virus. They tested how long the virus stayed alive under different acidity levels, measured by pH. At higher pH levels, between 5.8 and 6.6, the virus remained active through the entire cheesemaking process and continued to survive for up to four months during aging.
“This research was initiated due to previous work demonstrating high levels of virus shedding in milk from infected cows and the fact that we have previously shown that the virus survives in refrigerated raw milk for extended periods of time,” explained Dr. Diel.
When the milk was acidified to a pH of 5.0, the virus was destroyed early in the process. This means that the more acidic the cheese, the less likely it is to carry live virus particles.
Why This Discovery Matters for Public Health
The study’s findings are important because they show that aging raw-milk cheese for 60 days, which is the current FDA rule, does not guarantee safety. The bird flu virus was still active long after this period.
“The current regulation requiring 60 day aging of raw milk cheese before marketing proves insufficient to achieve H5N1 virus inactivation and guarantee cheese safety,” the researchers concluded.
Millions of people worldwide eat raw-milk cheese, believing it is healthier or more natural. In the United States, about 4 percent of adults say they drink raw milk, and 1.6 percent consume raw-milk cheese regularly. The popularity of these products means more people could unknowingly be exposed to infectious viruses if safety standards do not change.
Recent real-world cases show this concern is not theoretical. Several dairy workers and even household pets, such as cats, have become infected after contact with raw milk from sick cows. The virus’s ability to jump between species makes it especially risky.
Related Reading
Yoga Isn’t as Heart-Healthy as You Think. New Study Shows Exercise Works Better — A new study reveals that yoga offers many benefits, but when it comes to heart health, traditional exercise still outperforms it.
Read the Full StoryHow a Virus Can Survive Inside Cheese
At first, it may seem strange that a virus could survive in something like cheese. But the science behind it is clear.
The H5N1 virus can stay alive in cool, moist environments. When raw milk is turned into cheese, the process often involves mild heat and slow aging, not enough to destroy the virus. The study found that temperatures used during cheesemaking, usually around 40 degrees Celsius, are too low to kill H5N1.
Cheese also has water and proteins that protect the virus’s outer shell, helping it survive longer. The researchers compared three acidity levels and found that higher acidity, which is lower pH, helps break down the virus’s structure. That is why cheeses made with strong acid, like some yogurts or feta, are less likely to carry infectious viruses.
The scientists also tested whether eating contaminated cheese could cause infection. Ferrets, which are often used as models for human flu studies, were fed raw milk and raw-milk cheese. The ferrets that drank contaminated milk got sick. However, the ones that ate contaminated cheese did not.
This shows that while the virus can survive in cheese, it does not easily infect through digestion. Still, its presence means the product is not sterile and could pose a risk under certain conditions.
Who Should Be Paying Attention to This Study
This research should matter to everyone who buys or eats dairy, but certain groups need to be especially cautious.
- Consumers of raw-milk cheese: People who purchase unpasteurized cheese at farmers markets or from local producers face the highest risk.
- Dairy farmers and workers: Handling milk from infected cows can expose workers to high virus levels.
- Pregnant women, children, and those with weak immune systems: These groups are more vulnerable to infections from contaminated foods.
Even if the chance of infection through cheese is low, the study shows that the virus remains alive in conditions where it was once thought to die. For scientists and regulators, that is a strong signal that existing rules need review.
Why the 60 Day Rule Might Not Be Enough
For decades, the U.S. Food and Drug Administration has required raw-milk cheeses to age for at least 60 days before they can be sold. The idea behind this rule was simple. Time and acidity would naturally kill harmful bacteria and viruses.
But this new research suggests that the H5N1 virus can survive far longer than 60 days at cold temperatures. Even after 120 days of aging, small amounts of infectious virus were still detected in the cheese.
This finding means regulators may need to rethink how they define safe when it comes to raw-milk products. Scientists recommend that cheesemakers add an extra safety step, such as pasteurization, thermization, which is heating milk to around 54 degrees Celsius for 15 minutes, or acidification before production.
Without these steps, aged cheese made from infected milk could still carry live virus months later.
What This Means for the Dairy Industry
The dairy industry now faces a major challenge. Many artisanal cheese producers market their products as natural and unpasteurized, which appeals to consumers seeking authenticity. However, this same quality could make those cheeses unsafe during a bird flu outbreak.
“The work we have done on H5N1 is critical to providing practical, timely, data driven knowledge and recommendations to the dairy industry in the face of this outbreak,” said Dr. Nicole Martin, director of Cornell’s Milk Quality Improvement Program.
This study gives producers a clear message. Acidifying milk or lightly heating it before cheesemaking can destroy the virus without changing the product’s flavor or texture.
The researchers emphasized that this is not about banning raw-milk cheese but about making it safer. Countries like France and Italy, where raw-milk cheeses are part of the culture, may also need to assess similar risks if H5N1 spreads to dairy herds there.
A Global Food Safety Wake Up Call
The discovery goes beyond cheese. It raises new questions about how viruses behave in foods we consider stable or safe. Similar conditions exist in yogurt, whey, and other raw dairy products. The virus’s stability shows that refrigeration alone cannot protect against contamination.
This means that public health systems and food safety agencies around the world should update testing and handling procedures for raw milk. For consumers, awareness is the first line of defense.
What Everyday Consumers Should Know
The takeaway from this research is simple but vital. Aging does not kill the bird flu virus in raw-milk cheese. Only heat or strong acid can destroy it.
If you buy cheese, look for labels that say pasteurized milk. These products are safe because pasteurization kills bacteria and viruses. Avoid unpasteurized milk or cheese during outbreaks or if you do not know where it was produced.
Remember, even though ferrets in the study did not get sick from eating contaminated cheese, humans could still face risks, especially if the virus changes over time.
This finding is not meant to scare people away from dairy. It is a reminder that safety rules exist for a reason and that science can help improve them.
Related Reading
Your Eyes Have an Aging Switch, And Scientists Just Found How to Turn It Off, Scientists have identified a molecular “switch” in the eye that controls aging, opening new possibilities for reversing vision decline and restoring eye health.
Read the Full StoryFinal Thought
Cheese may age gracefully, but the viruses inside it do not always die with time. This new research from Cornell University shows that bird flu can live far longer than expected in raw-milk cheese. As scientists call for updated food safety measures, it is clear that our understanding of what makes cheese safe needs to change too.
The next time you pick up a wheel of raw-milk cheese, it is worth remembering. Sometimes, what you cannot see can stay alive much longer than you think.
FAQs
Yes. Cornell University researchers found that the H5N1 bird flu virus can survive in raw-milk cheese for up to 120 days, especially when the cheese has a higher pH (less acidic). Only strong acidification or heat treatment fully inactivated the virus during cheesemaking.
It’s best to avoid unpasteurized dairy products during outbreaks. The study confirmed that standard aging (60 days) does not destroy the virus. Pasteurized cheese, however, is safe since heat kills H5N1 and other pathogens. Always check for “pasteurized milk” on the label.
Producers can make raw-milk cheese safer by testing milk for H5N1 before production, pasteurizing or thermizing milk, and lowering the cheese’s pH through controlled acidification. These steps align with recommendations from Cornell’s Milk Quality Improvement Program and the FDA for reducing viral persistence in dairy products.