My Mom discovered koolaid can have this same effect when she rushed me to the pediatrician in horror at the color of the stuff in my diaper. This was the '70's, so I'm not sure if it's still true. My family identified several colors not found in the crayola box as things like "baby-poop yellow" "baby-poop green" etc..
I heard an interview yesterday on NPR with a guy who wrote a book about rats, and he said that if there's a plague scare and we kill off all the rats using some draconian poisoning program, the fleas that carry the plague will just jump from the rats to us.
Also, rats can chew through steel and concrete. But that's not important right now.