The word hepatitis means inflammation of the liver. It can be caused by a variety of organisms, drugs, exposure to toxic chemicals, and alcohol. However, virual hepatitis is the most common.

The viruses that cause hepatitis was first identified in the 1960s – 70s with the help of a study of children at the Willowbrook State School for the mentally Handicapped in Long Island, New York. Filthy living conditions in the school created an infection rate of almost 100%.

Viruses that cause hepatitis are named A, B, C, delta, and E. The hepatitis A virus (also called HAV) and the hepatitis B virus (also called HBV) are the most common.

The HAV closely resembles the poliovirus. It consists of a single strand of RNA enclosed in an icosahedral (20 sided) shell. It often shows up where sanitation and hygiene is poor. However, as with some viruses, early infection, which happen in poor countries, mean mild symptoms. Outbreaks of HAV in the U.S. have mostly occurred in day care centers, among gay men, and in connection with polluted water or uncooked food. For example, shellfish from contaminated water. It has been on a decline since 1971 when approximately 50,000 Americans were infected, mostly in the 15-24 age group. HAV also shows strong "socioeconomic" ties, or how rich you are in society. HAV infects both humans and primates. People are immune to the disease once they've caught it once.

The HBV however, infects only humans, even though they have close relatives which infect ducks, woodchucks, and squirrels. HBV can stay dangerous on needles, surgical tools, thorns, and sharp stones. In Asia and Africa, HBV is commonly passed during childbirth or breast milk. It is also ranked as the world's most common virus to cause cancer since it often brings with it liver cancer. Some people (5% to 10%) who are exposed to HBV will not become seriously ill, but will still be capable of infecting others. There is an estimated 400,000 to 800,000 carriers in the U.S. and 150 million to 200 million carriers worldwide. HBV has been on a decline too. It fell 59% between 1985 and 1993. A vaccine for HBV was developed in 1982.