More than 100,000 people in the United States have become allergic to red meat since 2010 in reaction to tick bites, the government reports.
Medical researchers say they believe many more people have the allergy and do not know it.
The National Centers for Disease Control and Prevention released two reports on the subject recently.
Ticks are very small animals that feed on the blood of other animals.
One report estimated that as many as 450,000 Americans have developed the meat allergy.
That would make it the 10th most common food allergy in the U.S., said Dr. Scott Commins.
The University of North Carolina researcher co-wrote both reports.
Health officials said they do not know of any confirmed deaths.
However, people with the allergy have described it as strange and terrifying.
"I never connected it with any food because it was hours after eating," said one patient, Bernadine Heller-Greenman.
The reaction, called alpha-gal syndrome, happens when an infected person eats beef, pork, or other meat of mammals.
Taking in other mammal products, such as milk, also causes the reaction.
A sugar called alpha-gal exists in mammal meat and in the saliva of ticks.
When the sugar enters the body through the skin, the body's own defense, or immune, system reacts too strongly.
Scientists saw such reactions in patients taking a cancer drug that was made in mouse cells containing the alpha-gal sugar.
But in 2011 researchers first reported that it could spread through tick bites, too.
They tied it to the lone star tick.
Scientists say the ticks live mainly in the eastern and southern U.S.
One of the studies released recently examined test results from the main U.S. private laboratory looking for alpha-gal antibodies.
Those tests showed the number of people who had the antibodies rose from about 13,000 in 2017 to 19,000 in 2022.
Experts say cases may have increased for several reasons.
They say the lone star tick population is expanding.
They also suggest more doctors might be learning about the disease and ordering tests for it.
But many doctors are not.
The second study was based on a survey last year of 1,500 U.S. general care doctors and health professionals.
It found nearly half of them had never heard of alpha-gal syndrome, and only five percent said they felt very sure they could identify it.
Researchers used that information to estimate the number of people with the allergy — 450,000.
People with this condition can experience symptoms including hives, nausea, vomiting, diarrhea, severe stomach pain, difficulty breathing, and swelling of the lips, throat, tongue or eyes.
Unlike some other food allergies, which happen soon after eating, these reactions hit hours later.
Some patients have only stomach symptoms.
The American Gastroenterological Association says people with unexplained diarrhea, nausea and stomach pain should be tested for the syndrome.
Doctors advise people with the allergy to stop eating meat and other foods that cause the reaction and to have epinephrine available to them at all times.
And, doctors say, avoid tick bites.
The allergy can go away in some people.
Commins said he has seen that happen in about 15 to 20 percent of his patients.
Staying free of tick bites is extremely important.
"The tick bites are central to this. They perpetuate the allergy," he said.
One of his patients is Heller-Greenman, a 78-year-old art historian in New York.
She spends summers on Martha's Vineyard, Massachusetts.
She said she is used to getting bitten by ticks on the island.
She has had Lyme disease, also caused by tick bites, four times.
About five years ago, Heller-Greenman started experiencing terrible, itchy hives on her back, middle and legs in the middle of the night.
Her doctors decided it was an allergic reaction but could not identify the cause.
She never ate a lot of meat, but one day in January 2020 she had two beef meals in a row.
Six hours after eating the second, she woke up nauseated, then vomited, had diarrhea and a lack of balance, or dizziness.
She passed out three times.
Doctors identified alpha-gal syndrome as the cause soon after the incident.
Heller-Greenman was told to avoid ticks and to stop eating red meat and milk products.
There have been no allergic reactions since.
"I have one grandchild that watches me like a hawk," she said, making sure she reads food labels and avoids foods that could cause a reaction.
"I feel very lucky, really, that this has worked out for me," she said.
"Not all doctors are knowledgeable about this."
I'm Andrew Smith. And I'm Caty Weaver.