Pus is the detritus of the death of certain types of white blood cells as the result of fighting infection. In the normal course, pus is taken away in the bloodstream and processed by the liver. However, in many cases, it can accumulate at the site of the infection, forming an abcess. The abcess can act as the site of a secondary infection, with the pus essentially providing nutrients for the new infection.

If pus is in an accessible place, it can be drained from the abcess. However, with the advent of antibiotics, the current treatment is to use antibiotics to treat the secondary infection. This will often stop the production of pus as the infection subsides and allows it to once again be processed in the bloodstream.

However, if the abcess is just below the surface of the skin, a physician will often lance the abcess to allow the pus to drain.

Pus at Wikipedia