What is the reasoning behind perspiration that smells bad? for example "onion"

Sweat : Sweat itself has very little odor. Sweat is the way that our bodies cool themselves, because as it evaporates our skin becomes cool. It is normal to sweat more if you are nervous. The odor in sweat comes from the bacteria that live on our skin. The bacteria thrive in the sweaty areas, as bacteria do well in moisture, and can survive on the oils that are in the sweat. The bacteria and their products are what give sweat a smell. Tasty. Bacteria also love moisture, and it aids in bacterial reproduction. As the bacteria feed and reproduce, they give off a smell that we associate with sweat.

Bacterial colonies: Normal sweat is non odorous and simply has a salty taste from loss of salt from the skin. If it is mal-odorous, it means that bacteria are growing and forming colonies. Frequent baths with anti-bacterial soap should take care of this. Also certain bacterial skin infections can cause malodorous sweat.