An alligator.
THE PUNCHLINE OF THIS JOKE HAS BEEN CENSORED BY THE GOVERNMENT OF THE PEOPLES REPUBLIC OF CHINA, PLEASE REMAIN CALM WHILE WE WILL DEAL WITH THE OP IN A CIVIL MANNER.
Because it was too chicken!
Because they were vaccinated as children.
An illogical situation imagined by a child.
An investigator.
An in-vest-igator!