Heterogeneity is a feature of all aspects of HDV infection and biology. Epidemiological studies indicate wide variations in the global distribution of the virus, often with different and bizarre rates of prevalence within apparently homogeneous populations. The medical expression of the infection is likewise manifold, ranging from the asymptomatic carrier of HDV to terminal cirrhosis. The clinical variability is matched at the biological level by extensive molecular heterogeneities, as expected with a small RNA virus such as the HDV.