Trimethylaminuria is a rare metabolic disorder that causes a defect in the normal production of a specific enzyme. Long story short, when the body can't break down trimethylamine during food digestion, the compound builds up and is released in the person's sweat, urine, and breath, giving off a strong fishy body odor.

The odor varies from person to person; in one, it will just smell like terrible body odor, but in most people it gives off a noticeable pungent fish odor.