The Lone Star Tick has been causing a human allergy to alpha-gal which are protein-linked saccharides found in meats. While people had been reporting meat allergies for several years it took a second group of people taking a drug called cetuximab reporting the same symptoms in the same geographic region to launch a full on investigation.

RadioLab has a great personal account that also explains the discovery of this allergy.

Via Wired:

Over the next few years Platts-Mills and his colleague Scott Commins screened more meat allergy patients and discovered that 80 percent reported being bitten by a tick. What’s more, they showed that tick bites led to a 20-fold increase in alpha-gal antibodies. Since ethics standards prevented them from attaching ticks to randomized groups of patients, this data was the best they could do to guess how meat allergy arises. Something in the tick’s saliva hijacks humans’ immune systems, red-flagging alpha-gal, and triggering the massive release of histamines whenever red meat is consumed.