And do the wittle bunny wabbits that hang out in occupied prairie dog holes, not get oralsyphyilitic hirsuta's and tuberculocitae cynicalmurisal's? And do the PD's in the rabbit infested holes not pick up their spiltsyphilus cunilingualcuti's? And do the coyotes that eat 'em both, not get either?
Now I'm more cornfuseder than ever.....
Many flea are specialist, designed to live in one ecosystem, and each different species of critter is kind of like a different ecosystem. Same thing with many germ strains. There is a version of HIV that only infects [bleep], but doesn't kill them.