The smell of rotting fish is hard to ignore, and should especially be a red flag if you notice it in your pee. If your toilet bowl smells like a fish market gone bad, it could be trimethylaminuria (try saying that three times fast, or just one time, slow). It's a metabolic disorder in which your body can't break down trimethylamine, a compound that gets synthesized by bacteria in your gut when you eat foods high in choline and carnitine, like eggs and red meat.
If this sounds horrible, it is. This one's also a rare genetic disorder, but for those afflicted, the odor can also impact breath and sweat, according to Dr. Ellerkmann, so it's hard to get away from. It's probably best to discuss with your doctor to see how best to manage it.
PKU, short for phenylketonuria, is another inherited disorder where the body can't break down phenylalanine, an amino acid found in protein. "If PKU is untreated, or if foods containing phenylalanine are eaten, the breath, skin, ear wax, and urine may have a 'mousy or 'musty' odor," Dr. Ellerkmann says. This unpleasant smell is from the buildup of phenylalanine in the body, and if untreated, could lead to serious health complications such as intellectual disability, seizures, and brain damage. But don't let this freak you out too much -- PKU is usually diagnosed in infancy, so if you haven't developed it yet, you're probably good.