Can someone please explained what causes this or how to stop it from happening!

Usually lack of a proper vent through the roof or a blocked vent (leaves, snow) will produce a smell of decay.
Bacteria is not an issue. Bacteria is everywhere. You want clean utensils and cutting boards, mainly because of uncooked meats, but you can't keep bacteria out of a drain. If your drain is draining, I doubt that you have mold. Again - you smell rotting food.

Which drain or drains? Kitchen, bath, shower? Is there a shared vent out the roof? Do you have a ton of snow where you are?

Which sink drain; kitchen, bath, all...

Food particles, skin, hair, other items that support microorganisms. Bleach, scrub with a brush, very hot water to reduce but never eliminate, bacteria is everywhere.