If he's actively vomiting and diarrhea don't feed but give small amts. of fluid. Vet might have to subcu some or show you how.
Giardia is possible and yes contagious, it's an organisim, but it's fairly simple to treat with the right meds.
I'm concerned about parvo more, as well.
If he had worms pretty bad, the ivermectin might could do it though...hm...in the process of the worms dying that is...I don't know for sure!