Wikipedia states the cause of Stinky feet or odiferous feet is quoted as follows:

The main cause is foot sweat. Sweat itself is odorless, but it creates a beneficial environment for certain bacteria to grow and produce bad-smelling substances. These bacteria are naturally present on our skin as part of the human flora. Therefore, more smell is created with factors causing more sweating, such as wearing shoes and/or socks with inadequate air ventilation for many hours. Hair on the feet, especially on the toes, may contribute to the odor’s intensity by adding increased surface area in which the bacteria can thrive.

Given that socks directly contact the feet, their composition can have an impact on foot odor. Polyester and nylon are common materials used to make socks, but provide less ventilation than cotton or wool do when used for the same purpose. Wearing polyester or nylon socks may increase perspiration and therefore may intensify foot odor. Because socks absorb varying amounts of perspiration from feet, wearing shoes without socks may increase the amount of perspiration contacting feet and thereby increase bacterial activities that cause odor

One in six people suffers from a foot odor problem on a daily basis.

It is one of the most overlooked problems in personal hygiene because most people don’t know how to take control of it.

Foot odor is normally viewed as a relatively minor problem and many healthcare professionals often brush it off as something that you have to just learn to live with.

It’s not seen for what it really is, which is an indication that a person’s natural bacterial balance has changed on the surface of their skin.

When there is an excess of “bad” bacteria in relation to “good” bacteria, a foul-smelling odor is produced because the “bad” odor-producing bacteria has found an ideal place on your body to multiply and flourish.  This can lead to Athlete’s foot.