Hepatitis is an inflammation of the liver. Hepatitis can be caused by an infection with a virus, but poisonous (toxic) substances can also cause it. Researchers have identified several of the viruses responsible for hepatitis, however some patients with hepatitis show no evidence of being infected with known hepatitis viruses. Researchers call conditions like this, seronegative hepatitis. It means that a patient has hepatitis but he/she does not have evidence in their blood of a viral infection.