LONDON—A mysterious syndrome in which men come down with a flu-like illness after an orgasm may be caused by an allergy to semen, Dutch scientists say.

Men with the condition — known as post-orgasmic illness syndrome or POIS and documented in medical journals since 2002 — get flu-like symptoms such as feverishness, runny nose, extreme fatigue and burning eyes immediately after they ejaculate.

Symptoms can last for up to week.