Muzan, the demon king created a virus out of the red spider lily, in spite that he couldn’t find the blue spider lily so the virus affects demon slayers/humans, the symptoms: spasms and 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 they rot away.