I have an idea for a crazy super-organism which could wipe out the human race. Essentially, it is an organism (plant or fungus or bacterium depending on what would be more effective or what would be possible) which spreads itself through the atmosphere.
Upon managing to find its way into the human body, it replicates itself. It may stay in the host for an indefinite length of time, or it may begin to poison the human in such a way that the human becomes unable to move (it could just kill them). Once the human is unable to move, the parasite begins to replicate itself on the outside of the body. It essentially uses the body to build a point of emission for its method of transmission, then spreads itself using the incapacitated human as a 'base camp'.
Is this possible, as far as biologists are aware? I've left a lot of things open here on purpose - the things which I need kept consistent are the method of transmission and the whole incapacitation thing.