An equine abcess is a place of concentrated bacterial infection in the body of the horse. Normally it associated with the production of large amount of pus. Pus is created by a mixture of dead and dying white blood cells from the body, dead or dying bacteria and decomposing body cells. Because of all the cells that are dying within an abcess it contains many toxins and these toxins generally leak out making the animal feel unwell.
It is normally important to drain an abcess quickly to enable to toxins to escape. Anbiotics are not very effective at treating abcesses until they are drained because they cannot penetrate the wall of the abcess that does not have a blood supply. When abcesses are in places like the lungs they may take a long time to clear up and will recur because the infection becomes walled off but not totally irradicated.
Immune boosting products can help antibiotics clear up abcesses more effectively.