> What? How could a cat pass anything on to a rat?

Everybody poops.

Everybody eats and drinks.

Sometimes these two overlap more than is sanitary.

It's not a virus, but check out the parasite toxoplasma gondii for an example of an organism that uses this mechanism to get from cats (it only can reproduce in cat intestines) to rats (where it lives in their brains).

It needs to get back to cats from the rats when it is time to reproduce, which is the really interesting part of its life cycle. It doesn't just wait around on the off chance a cat will happen to eat the rat. No, it alters the rat's brain to make the rat lose its fear of the smell of cat urine. Normally, rats avoid territory marked by cats, but not those with a toxoplasma gondii infestation. It even makes male rats confuse that smell with that of a female rat, so they actually end up seeking out cats.

Truly something out of a horror movie [1].

PS: it's also not uncommon in humans.

[1] https://www.smithsonianmag.com/science-nature/the-parasite-t...

My cat plays with the rats and mice she catches before she kills them to eat. Even then she rarely eats all of the animal. Brains seem to be her preference since she eats the heads and leaves most of the rest on the porch. Some of the small animals she catches escape because she gets bored or distracted when someone comes out the door to see what all the commotion is about.

With that said, you're right, it doesn't make any sense that a rat would live long enough to catch anything in an encounter with a cat.