What causes eye styes?

According to WebMD, an eye stye is often caused by a staphylococcus bacterial infection of the eye. A stye can be very painful and cause a pus-filled lump to form on the inside or outside of the upper or lower eyelid.

WebMD explains that styes can be external or internal. An external stye usually starts on the lower eyelid next to an eyelash. This type of stye turns red and can swell for many days before opening up and healing. External styes are self-limiting and can disappear after several days. An internal stye is located on the underside of the eyelid. This type of stye causes a red, swollen lump to form but it is not always self-limiting and can cause a cyst to form. A cyst needs to be drained by a doctor, according to WebMD.