Is Panleukopenia virus really that dangerous to cats?

This is related to my case about my 2 cats that has been admitted to veterinary hospital for days now. The doctor just contacted me and say one of my cat ped away 10 minutes ago because her antibody is too weak to handle the virus while my other cat is still in critical condition. The other cat that survive until now is a male cat and he has been dripping until today. I'm really scared that i would lose him too...so can anyone here tell me how serious Panleukopenia virus to cats? I read some informations from the net it says it's a type of virus that transfered from other cats and i rescued a kitten home recently from the street, is it because of the kitten that made my 2 cats fell ill? Could someone explain this to me please?