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Illustration of hepatitis B virus (HBV) particles. HBV causes the disease hepatitis B (hep B), which affects the liver. Hep B can cause acute symptoms lasting several weeks, from which most recover naturally, but chronic hep B can cause cirrhosis and liver cancer to develop over many years. There is no cure for chronic hep B, but vaccines exist which are offer near 100 percent protection from infection. (KEYSTONE/SCIENCE PHOTO LIBRARY/NATIONAL INSTITUTES OF HEALTH, NATIONAL INSTITUTES OF HEALTH)