Muzan, the demon king made a virus based on the spider lily, the infected begin to spasm and have urges to cannibalise, spider lily-like tendrils begin to grow out of their skin, eyes, nose and mouth. The teeth sharpen and they become hostile, if you are bitten or any wound is exposed to the virus, it infects you and the symptoms begin to show shortly after, the bodies have stages to where eventually it completely takes over and the body decays beyond repair, leaving a decrepit moving corpse.