The bacterium that is responsible for boils is called staph bacteria (scientific name: Staphylococcus aureus). Normally, the bacteria are found in your skin, throat and nasal passages. they are responsible for several serious diseases like pneumonia, endocarditis, meningitis and urinary tract infections. Several food-borne and hospital infections are also caused by them. When your skin gets cut or scratched, staph bacteria can enter it. The moment this happens, your body's immune system kicks in, sending the bacteria-killing soldiers—the white blood cells, called neutrophils—to kill the infection. The battle that ensues is what causes the boil to get inflamed; eventually, pus forms. Pus is a mixture of bacteria, dead skin cells and white blood cells.
Ouch! I just had one in my armpit and i thought that was bad. I just left it alone, eventually it will pop, a bunch of nasty green $hit ran out, and a few days later it disappeared. I put some antibacterial ointment on it after it popped.