PARENTERAL TRANSMISSION: Among subjects infected by the hepatitis C virus (HCV), about 40% have no history of blood transfusion or intravenous drug abuse. The highly variable presence of HCV in biological fluids other than blood would suggest that HVC transmission basically follows the parenteral route. Transmission of HCV via medical material contaminated by blood of an infected subject is a clinical reality: accidental needle prick, medical material (endoscope, physician-patient), tattooing, acupuncture, ear piercing, certain traditional practices, sharing toilet instruments (tooth brush, razor, fingernail shears). RARE SEXUAL TRANSMISSION: The prevalence of HCV infection is higher in people living with infected subjects, particularly spouses, than in the general population. However, transmission of HCV in this population probably follows a parenteral route (common risk factors, sharing toilet instruments) rather than by sexual transmission which plays a minor role except in sexually transmitted diseases with genital lesions. MOTHER-INFANT TRANSMISSION: Per- or post-partum transmission is possible though the risk is low, less than 5% of all infants are infected at the age of 1 year. The data are contradictory, but breast feeding would appear to play a role. Co-infection by the HIV virus, via high HCV viremia, clearly increases the risk of mother-infant transmission and perhaps also sexual transmission. NOSOCOMIAL TRANSMISSION: Nosocomial transmission is probably the most important factor in HCV transmission, but the risk remains to be quantified.