Infection may originate from the patient (endogenous), usually from skin, nasopharynx or bowel, or from outside sources (exogenous), often another person who may be either suffering from an infection or carrying a pathogenic microorganism. Carriers are usually healthy and may harbour the organism in the throat (for example, diphtheria or meningococci), bowel (salmonella) or blood (hepatitis B or cytomegalovirus).