In a stem cell research facility a bereaved staff attempts to bring back his wife, he uses an evolved version a parasite that controls animals to their death to control his wife to act as if she were hman. After she had been reanimated she attacked her husband, he didn't know for the parasite he had created to complete it's life cycle it needed to merge with another Human brain.
How it Spreads
On impact (bite) the parasite duplicates itself, to attack the other human. during the attack one of the duplicates moves to other host infecting it indefinitely.