Opportunistic infections are becoming really or apparently more frequent because of improved diagnostic techniques, increased age of the people in the developed world, better medical therapy, and higher number of patients living with transplanted organs and on immunosuppressive and glucocorticoid therapy, and also because of AIDS pandemic. Prevention of bacterial, fungal and viral infections includes classical preventive measures, surveillance, and education of patients and medical personnel. Vaccination is important especially to prevent viral diseases and also some bacterial ones. Routine antibiotic prophylaxis decreases some surgical infections. Meticulous follow-up of immunocompromised patients and specific anti-infective therapy saves the patients' lives in non-fatal diseases and prolongs the life in AIDS patients.