Odorous TMA is produced by microbes such as ocean-dwelling bacteria. Individuals suffering from Trimethylaminuria (TMAU) have malfunctioning enzyme flavin containing monooxygenase 3 (FMO3; 136132) and are unable to efficiently convert choline-derived trimethylamine into trimethylamine oxide. Trimethylamine then accumulates and is released in sweat, urine, and breath, giving off a strong fishy odor.
Dimethylglycinuria or dimethylglycinemia is another condition responsible for rotten fish odor. It happens when another enzyme processing choline - Dimethylglycine dehydrogenase (DMGDH; EC 188.8.131.52) - is not properly functioning. Dimethylglycinuria leads to increased production of dimethylglycine (DMG) in blood and urine, but this metabolite does not seem to be the source of the odor.
One of the symptoms of infectious disease Cholera is "rice-water stool" diarrhea that smells "fishy."