A really neat manga by Hitoshi Iwaaki. It tells the tale of unknown spores falling to earth and infecting humans and animals. These spores can morph the part of the human body they infect into different forms.
The hero of the story, shin, manages to avoid getting his brain infected. The strange being takes residence in his left hand. The two must survive together and figure out the mystery of where the spores came from. Lots of gore and badass art.