An abcess forms when bacteria from dirty gear, dirty syringe, etc enter the muscle/skin.
The body's antibodies (white blood cells) surround the infection in an attempt to
contain it from spreading further. The dea whit blood cells are what becomes the sub-
cutaneous pus. The sensitivity associated from an abcess is due to the pressure from
the fluid pressing down on the pain receptors, causing them to stimulate action poten-
tails more frequently, hence the pain at the slightest touch. most will go away over time,
but some need to be drained by a doctor.