An infection inside the tip of the finger can form an enclosed pocket of pus (or abscess) that is very painful as it expands. A felon is a fingertip abscess deep in the palm side of the finger. It usually is caused by bacterial infection, but a herpes virus called herpetic whitlow and, more rarely, fungi also can cause felons.
Felons usually are caused by Staphylococcus aureus bacteria. It is important to get treatment quickly for these small abscesses. If the bacteria pocket continues to expand, it can compress blood vessels in the finger and cut off the fingertip's circulation, causing in permanent damage to the finger. Also, an untreated felon can spread its infection to the bone within the finger. This can lead to a more serious infection, called osteomyelitis, which takes much longer to cure.
Like other types of infections, felons sometimes begin after the finger is punctured by a wood splinter or something else.