H just rang and said he was on his way home and he had a passenger. It turns out that the passenger is a kitten.
Apparently it's mother died and it spent the night crying outside his window so he went and got it and it was covered in feaces and fleas so he's washed the **** off and he's bringing it home Because he couldn't take it to the RSPCA because they would kill it

I've never, ever had a small kitten before. I've always bought from breeders who let them go at 3 months and they are vaccinated, wormed, and never covered in **** and fleas.
And I already have a cat and this is where it gets complicated -Rosie, the Queen of Sheba has FIV. So if she bites the kitten the kitten will get sick but if the kitten makes her sick she will get extra sick

And I'm busy this afternoon.

I've left a cat box by the door and told H to dump his dirty kitten in there and I'll deal with it but I have no idea how to deal with it. What does a kitten the size of your fist eat? What flea stuff can I use on it?
How do I keep ****ty kitty and Queen Rosie apart? Whilst still treating both humanely.