I saw this the other day... it's a fungi that infects ants and infects others by controlling the ant to go upwards. It even spurts tentacles and disgusting-looking deformities veeeeery much like Halo's Flood. It also adapts quickly to other insect species and mutates them as well. Now if this got out of control and mutated to a form able to overtake a human?
There's also a video game coming out called the Last of Us that explores the possibility of this infecting humans. Looks interesting.