8. Hepatitis B What it is: A chronic liver disease caused by the hepatitis B virus. How you get it: People with hepatitis B have the virus in their blood and other bodily fluids. Adults usually spread it through sex or sharing needles. A pregnant woman can pass it to her baby. Hepatitis B is 100 times more infectious than HIV, the disease that causes AIDS. Why it’s serious: It can lead to liver cancer and other long-lasting liver diseases, which can be deadly.