FIRST COMES THE unscratchable itching, and the angry blossoming of hives. Then stomach cramping, and—for the unluckiest few—difficulty breathing, passing out, and even death. In the last decade and a half, thousands of previously protein-loving Americans have developed a dangerous allergy to meat. And they all have one thing in common: the lone star tick.

Red meat, you might be surprised to know, isn't totally sugar-free. It contains a few protein-linked saccharides, including one called galactose-alpha-1,3-galactose, or alpha-gal, for short. More and more people are learning this the hard way, when they suddenly develop a life-threatening allergy to that pesky sugar molecule after a tick bite.

Yep, one bite from the lone star tick—which gets its name from the Texas-shaped splash of white on its back—is enough to reprogram your immune system to forever reject even the smallest nibble of perfectly crisped bacon. For years, physicians and researchers only reported the allergy in places the lone star tick calls home, namely the southeastern United States. But recently it's started to spread. The newest hot spots? Duluth, Minnesota, Hanover, New Hampshire, and the eastern tip of Long Island, where at least 100 cases have been reported in the last year. Scientists are racing to trace its spread, to understand if the lone star tick is expanding into new territories, or if other species of ticks are now causing the allergy.

The University of Virginia is deep in the heart of lone star tick country. It's also home to a world-class allergy research division, headed up by immunologist Thomas Platts-Mills. He'd been hearing tales of the meat allergy since the '90s—people waking up in the middle of the night after a big meal, sweating and breaking out in hives. But he didn't give it much thought until 2004, when he heard about another group of patients all suffering from the same symptoms.