Jerry Groopman in The New Yorker:
Several years ago, I fell at the gym and ripped two tendons in my wrist. The pain was excruciating, and within minutes my hand had swollen grotesquely and become hot to the touch. I was reminded of a patient I’d seen early in medical school, whose bacterial infection extended from his knee to his toes. Latin was long absent from the teaching curriculum, but, as my instructor examined the leg, he cited the four classic symptoms of inflammation articulated by the Roman medical writer Celsus in the first century: rubor, redness; tumor, swelling; calor, heat; and dolor, pain. In Latin, inflammatio means “setting on fire,” and as I considered the searing pain in my injured hand I understood how the condition earned its name.
Inflammation occurs when the body rallies to defend itself against invading microbes or to heal damaged tissue. The walls of the capillaries dilate and grow more porous, enabling white blood cells to flood the damaged site. As blood flows in and fluid leaks out, the region swells, which can put pressure on surrounding nerves, causing pain; inflammatory molecules may also activate pain fibres. The heat most likely results from the increase in blood flow.
More here.