In the developed world, the most common way that people get Hepatitis C is through intravenous drug use (by shooting drugs into a vein, using a needle that was already used by a person who has Hepatitis C). In the developing world, most people get Hepatitis C through blood transfusions (being given blood that was taken from a person with Hepatitis C) or by getting medical care with tools that were not cleaned enough after being used on a person with Hepatitis C. In 20% of all cases of Hepatitis C (or 1 in every 5 cases), it is not known what caused the infection, but many of these cases are thought to have been caused by intravenous drug use.