Hepatitis D virus (HDV) is a highly defective virus since it cannot produce infective virions without the help of a co-infecting helper virus. This helper virus is hepatitis B virus (HBV) that supplies the proteins of the superficial membrane (HBsAg). In budding out of the cell, HDV acquires a membrane containing HBsAg. The outcome of disease largely depends on whether the two viruses infect simultaneously (coinfection), or whether the newly HDV-infected person is a chronically infected HBV carrier (superinfection). Coinfection of HBV and HDV results in both acute type B and acute type D hepatitis and superinfection is associated with fulminate acute hepatitis and severe chronic active hepatitis, often progressive to cirrhosis. Detection of HDV RNA by PCR allows detection of the causative agent in the period of introduction of infection before seroconversion, which is very important for early diagnostics.