Pimples, also known as acne, are primarily caused by the overproduction of oil, blockage of hair follicles, bacteria, and inflammation. When oil glands produce excessive oil, it can mix with dead skin cells and clog pores, creating an environment for bacteria to grow. The immune system responds to these clogged pores, resulting in inflammation and the formation of a pimple. Other factors such as hormonal changes, certain medications, diet, and genetics can also contribute to the development of pimples.