What usually causes prostatitis?

Inflammation. There is 4 general catagories of prostatitis: 1. Acute bacterial - caused by infection 2. Chronic bacterial - recurrent bacterial infection 3. Chronic pelvic pain syndrome - may result from chronic nonbacterial prostatitis - most common (inflammation without infection) or prostadynia (symptoms without infection or inflammation) 4. Asymptomatic inflammatory prostatitis - found on biopsy.
Multiple factors: bacterial infection, excessive intercourse/masturbation/oral sex/manipulation. Requires a professional examination and analysis of prostatic fluid plus a digitial rectal exam.