There is a virus known as the demon virus, basically it makes the person burst out with a lot of demon faces on your body with teeth that like to eat human flesh... but thats just in the first stages of the virus. The main guy Yamata, I think, has been infected with the virus.. except that he is immune to the virus.. but can transform into a large creature that kicks butt. Anyway the guys friend is trying to find a cure for it, but while they are trying to find one, many people are dying. He starts wondering how long he can keep the virus under control and he loses it later on and he dies fighting another infected person, I believe.