Hepatitis D virus (HDV) depends on the synthesis of the hepatitis B surface antigen (HBsAg). Without the HBsAg coating, HDV cannot infect on its own. HDV infects on average 4 percent of acute hepatitis B cases in the United States. Can be acquired either as a coinfection or as a superinfection in persons already infected with HBV. The incubation period ranges from three to seven weeks. Symptoms are similar to HBV but can be more severe. 70 - 80 percent of chronic HBV carriers with HDV superinfection develop evidence of chronic liver disease with cirrhosis, compared to 15 to 30 percent of patients with chronic HBV infection alone. The routes of transmission are similar to those for HBV (except perinatal transmission is rare). Individuals at risk for HDV are people with HBV, anyone at risk for HBV, injecting drug users, hemophiliacs/hemodialysis patients, homosexuals and heterosexuals with multiple sex partners. There is no vaccine specific for HDV but since HDV is dependent on HBV for replication, the most important tool for preventing HBV-HDV coinfection is immunization with hepatitis B vaccine.