Exercise-induced anaphylaxis was first described in 1979, and probably affects around 50 in every 100,000 people. While awareness of the condition among allergists has gone up, researchers and doctors still don’t know exactly why it occurs.

For between 30 to 50 percent of people, the reaction comes from combining certain types of food and exercise. For others, strenuous activity triggers a reaction to drugs like aspirin. Some women only experience the phenomenon when they’re at the point in their menstrual cycle with high levels of the hormone estrogen, because it can bind to the cells involved with an allergic reaction.

The amount of exercise needed to trigger a reaction varies from person to person. It generally takes more to cause anaphylaxis in someone who is relatively fit than someone who isn’t in great shape. Almost all types of exercise, like running, dancing, or biking, have been reported to cause anaphylaxis—though there haven’t yet been any reports of anyone having a reaction after swimming.

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