What such an account fails to address, however, is why the meat allergy has increased in other parts of the world, like Australia and Europe. (Van Nunen says that in the tick country around Sydney, people are now more likely to carry EpiPens, which contain a shot of adrenaline, for meat allergy than for better-known peanut allergies.) Other tick species are linked with meat allergy in those regions, not the lone-star tick. And it seems very unlikely that the microbiomes of all these ticks on different continents have changed in similar ways at the same time. “I don’t for the life of me have a unifying hypothesis for why it’s happening everywhere,” Commins told me, although he added that pesticides could be one factor changing tick microbiomes globally.

It may simply be that an increase in the number of ticks has turned a problem once so rare that it went scientifically unnoticed into an observable epidemic. “I think we’ve got far more tick bites today than people had as recently as 35 years ago,” Platts-Mills told me. He lays the blame for the growing spread of ticks on newly abundant deer. In Virginia, he thinks new laws requiring dogs to remain on leashes have emboldened deer, which then bring ticks closer to people. People aren’t necessarily venturing deeper into the forests than in the past, he says. More than half the patients he sees with the allergy were bitten on their own lawns.

His leash theory is anecdotal, but it’s certainly true that the current ecological state of Eastern forests is probably encouraging ticks to multiply. After having been cleared in the Colonial era, the forests have partly grown back. Deer and turkey, which the lone-star tick likes to feed on, are abundant again. They thrive in the new-growth forests, now fragmented by roads and suburbs. Large predators are mostly absent. And the rise of tick-borne disease generally has been linked with the decline (or absence) of predators that eat the animals ticks feed on. In Australia, for example, van Nunen points to the eradication of foxes, an introduced species there, as one factor in the increase of ticks and the rise of meat allergy.

We might label this the disturbed ecosystem theory of meat allergy. Forests ecosystems have recovered partially — lots of animal hosts for ticks but not enough predators to keep those hosts in check — and this imbalance has fostered an exponential growth in the number of ticks. In some ways, this is the most probable explanation for the rise of meat allergy. Climate change may be aiding the lone-star tick’s move northward too, Rick Ostfeld, a disease ecologist at the Cary Institute of Ecosystem Studies, told me. Hundreds of cases of meat allergy have been diagnosed on Long Island in recent years, which wasn’t part of the tick’s range in recent history. The tick has been spotted as far north as Maine.

But what’s happening in the American East can’t account for the full extent of the phenomenon elsewhere in the world. In Northern Europe, ticks are proliferating as forests recover and the climate becomes warmer. But in Spain and Southern Europe, the rising incidence of meat allergy has not been accompanied by an increase in tick numbers, according to José de la Fuente, a professor at the Institute of Game and Wildlife Research in Ciudad Real, Spain. For him, the mystery of meat allergy is captured in one question: If a tick bites two genetically similar people, why might only one develop the meat allergy?

Onyinye Iweala, an assistant professor who works with Scott Commins’s lab at the University of North Carolina, echoes this uncertainty. Why are some people sensitized to alpha-gal — meaning they have allergic antibodies directed at the sugar in their blood stream — but never have an allergic reaction to it? This can happen in all allergies. You can have antibodies to, say, cat dander, yet never wheeze or sneeze around cats. Iweala suspects that sensitization to alpha-gal isn’t new. What’s changing is the proportion of people who, after sensitization, proceed to overt allergy. Something else in the environment, she told me, is likely pushing people toward full-blown meat allergy. Perhaps shifts in the microbes that live within us have somehow made us more easily sensitized by tick bite. As a model of how this might work, Iweala points to intriguing research on the interaction between malaria and the human microbiome that centers on alpha-gal.

OUR DISTANT ANCESTORS once made alpha-gal. Understanding why humans don’t could shed light on the meat-allergy mystery. Like other mammals, South American monkeys produce alpha-gal. Only Old World monkeys and apes (and humans) have lost the ability to make the sugar. Hence scientists deduce that the change most likely happened after New and Old World primates diverged from each other around 40 million years ago. One explanation for the disappearance of alpha-gal is that it was driven by some catastrophe, a deadly infection that afflicted Old World primates, perhaps, and as a result maybe these distant relatives of ours stopped being able to produce the sugar because doing so conferred an evolutionary advantage. The mutation that eliminated alpha-gal could have improved a primate’s ability to fight off an infection by enabling its immune system to more easily distinguish between its own body and some pathogen with alpha-gal.