Boils on the scalp form underneath the skin when bacteria infects one or more hair follicles, according to the Mayo Clinic. Boils typically start as pus-filled lumps that grow, become painful and eventually form a head that bursts to allow the pus to drain. They are most commonly caused by staphylococcus aureus bacteria.
Boils have the potential to infect anyone; however, there are certain risk factors that make some people more susceptible to boils than others, according to the Mayo Clinic. Close contact with someone who has a staph infection increases risk, as does having diabetes, which lessens the body's ability to fight infection. Chronic skin conditions also increase risk as damage to the skin's protective barrier makes it easier for the bacteria to enter into the body. Anyone with an impaired immune system is also at higher risk.
The Mayo Clinic explains that if only one boil is present, home care is recommended, but for more than one boil, it is necessary to visit a doctor's office for treatment. Any time that a boil becomes extremely painful in a short period of time, is more than 2 inches across, has not healed for two weeks or recurs, visit a doctor to resolve the issue that may be preventing healing of the boil.