(1) Autogenous Sources. The pathogenic organism may invade directly from the site at which it is growing – e.g. a commensal Staph. Pyogenes may cause a boil or the micro-organisms may spread locally to different sites on the body surfaces. Thus Strept. Pyogenes from the nasopharynx may cause erysipelas of the face, or Esch. Coli from the large bowel may pass from the anus to the urethra and invade the urinary tract. Alternatively the transmission may be by the blood stream, as when a tooth is extracted and Strept. Viridians from the mouth enters the blood and colonizes abnormal heart valves and produces bacterial endocarditis.