They are such tough little things! It definately needs a trip to the vet but in the mean time.... just a little water and some cooked well mushed chicken..... Make sure not to give it any milk, especially cows, it is torture to their tummys that young. The local rescue centre or vet may be able to sell you "Wombaroo" a milk replacer, plus bottle and teat..... if the kitten is under 7-8 weeks it would probably be a good idea to give it bottles for a while to try and make up for the poor babies rough start in life, if it was that dirty it will probably be riddled with worms etc as well, so it will need the extra nutrition :-)

Im with you on not knowing past that, both of my boys are purebred, and had the same start to life as your 13 weekers lol, but having my stud boy I have been exposed to kittens that need feeding as the mother cant/wont feed, they seem to do really well once they get the hang of it.

In regards to the FIV thing, definately keep your precious girl seperate, but considering how this kitty came to you, I wouldnt be surprised if its mother was feral/abandoned..... there is a good chance it has already been exposed to these nasties

I hope it all goes well and little ****ty kitty will be all better soon