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 abscess. The abscess 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 abscess. 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 abscess is just below the surface of the skin, a physician will often lance the abscess to allow the pus to drain.