Hepatitis B is a liver infection caused by the hepatitis B virus (HBV). HBV is transmitted when blood, semen, or another body fluid from a person infected with virus enters the body of someone who is not infected. This can happen through sexual contact; sharing needles, syringes, or other drug-injection equipment; or from mother to baby at birth. For some people, hepatitis B is a short-term illness, but for others, it can become a long-term, chronic infection. Risk for chronic infection is related to age at infection: approximately 90% of infected infants become chronically infected, compared with 2%–6% of adults. Chronic hepatitis B can lead to serious health issues, like cirrhosis or liver cancer. The best way to prevent hepatitis B is by getting vaccinated. Hepatitis B is a liver infection caused by the hepatitis B virus. Hepatitis B can range from a mild illness lasting a few weeks to a serious, lifelong illness. Acute hepatitis B is a short-term illness that occurs within the first 6 months after someone is exposed to the hepatitis B virus. An acute infection can range in severity from a mild illness with few or no symptoms to a serious condition requiring hospitalization. Some people, especially adults, are able to clear the virus without treatment. People who clear the virus become immune and cannot get infected with the hepatitis B virus again. Acute infection can but does not always lead to chronic infection. Chronic hepatitis B is a lifelong infection with the hepatitis B virus. Over time, chronic hepatitis B can cause serious health problems, including liver damage, cirrhosis, liver cancer, and even death.