Sick of Exercise or Is Exercise Making You Sick?

Some people suffer from a rare allergic reaction to exercise that can be life-threatening.
1:59 | 04/25/14

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Transcript for Sick of Exercise or Is Exercise Making You Sick?
Now, what sounds like the dream of every couch potato, a reason not to exercise. It turns out some people are actually, legitimately, allergic to exercise. And many people who don't even know it. ABC's Reena ninan has the story. Reporter: It's the ultimate excuse to avoid the gym. I'm allergic to exercise. But for Caitlin, it's no joke. I have dots on my arms. Bad stomachaches. Brain fog. Reporter: That's right. When the 20-year-old soccer player works out too hard, it triggers an allergic reaction that be so life-threatening, it could put an end to her soccer career. The university's team doctor watched it happen. She got swelling around the mouth. My core temperature rises. And that triggers cells, which causes me to go into anaphylactic shock. Reporter: Anaphylactic shock can kill you. Similar bombs begin with exercise. Including hives or skin irritation. Swelling of the mouth or face, and abdominal pain. For some, it's a food they may have eaten before working out. 25% of the patients have food plus exercise as the allergic trigger. The double-whammy. Reporter: Others may have a mild form of the allergy and not realize it may be dangerous. It may progress. Reporter: Trainers tried everything to help Caitlin, from cooling baths to cold showers. But nothing worked. Today, her allergy is kept in check with antihistamines and asthma medication. But she hasn't completely hung up her sneakers. She's now coaching soccer at a local club. For "Good morning America," Reena ninan, ABC news, New York.

This transcript has been automatically generated and may not be 100% accurate.

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