Human Hepatitis Delta Virus (HDV), which infects humans only in the presence of a co-infecting Hepatitis B Virus (HBV), is a subviral agent. This agent, along with HBV, may cause co-infection, superinfection and chronic infections. More than 10 million people worldwide are infected with HDV and its helper virus, HBV. Recently, changes in HDV prevalence occur; while it decreases in certain areas, some new foci of HDV endemicity appear. The transmission of HDV, like HBV, frequently occures by use of contaminated blood and its products, and less frequently by close contacts with body fluids and sexual contact. Currently, there is no efficient therapy for HDV infections, rendering prevention very important. The HBV vaccination, particularly for people at high risk, prevents HDV infections along with HBV.