Hepatitis is a disease that causes inflammation of the liver. While many things can cause hepatitis, it's frequently caused by being infected with a virus.
A virus is a tiny piece of genetic material that's surrounded by a protective coat. This genetic material acts as a blueprint for the virus to make more copies of itself inside your body. A virus has only one purpose, and that's to make as much of itself as possible. There are many types of viruses, such as those that cause the common cold. Each virus tends to infect a particular area of the body.
The virus that causes hepatitis is able to enter liver cells, and then use those cells to make more viruses. As more and more of the virus is made in the liver cells, they can become damaged and may even die.
There are several types of viral hepatitis -- such as hepatitis A, B, C, D, and E. A different virus causes each type, and they all affect the liver a little differently. In the United States, hepatitis A, B, and C are the most common.