Staph, a kind of bacteria, is most often to blame. You have staph on your skin all the time. But if it gets inside your body, say through a cut, it can cause problems.
These other things can also cause folliculitis:
Blockages from skin products, such as moisturizers with oils
A fungus
Hair removal, such as shaving, waxing, and plucking
Ingrown hairs
Other bacteria, such as the kind you might find in a hot tub
Some drugs, such as corticosteroids that are used to ease inflammation