There is a host of reasons that cause smelly feet. One of the most common is a condition called hyperhidrosis (excessively sweaty feet). As the foot sweats bacteria begins to grow and proliferate around the sweat follicles of the foot producing isovlaeric acid which is responsible for the foot odor. The sweat will then settle inside of footwear producing a combination of a smelly foot and footwear. However, there are a number of conditions that will increase the chances of smelly feet that include drugs, stress, and hormonal changes.