Originally viruses were grouped and named according to the disease they cause, even if they are not closely related. For example Hepatitis A is an RNA virus and Hepatitis B is a DNA virus, but they both cause liver disease. Nowadays, however, viruses are grouped by their shared characteristics in much the same way as plants and animals are. RNA viruses all use RNA as the infections 'code'. This RNA can be single stranded or double stranded. RNA viruses tend to be quicker to mutate as RNA isn't proofread like DNA. Some RNA viruses use the host cell machinery to turn their RNA into DNA which is more stable.
