http://www.muscletalk.co.uk/article-...nfections.aspx
The majority of abscesses are septic (i.e. caused by an infection) but sterile abscesses can also occur which are not caused by germs but by non-living irritants such as drugs. If an injected drug, especially oil based ones such as many anabolic steroids are not fully absorbed, it stays where it was injected and may cause enough irritation to generate a sterile abscess. Sterile abscesses are quite likely to turn into hard, solid lumps as they scar, rather than remaining pockets of pus2.
Superficial abscesses are readily visible and are red, swollen, painful and warm. Abscesses in other areas of the body may not be obvious and may produce only generalized symptoms such as fever and discomfort. A sterile abscess may cause only a painful lump, for example deep in the buttock where a shot was given. If the abscess is small (less than 1 cm or less than a half-inch across), applying warm compresses/hot soaks to the area for about 30 minutes 4 times daily can help.