[Carina43]

Vitamin C is a great immune booster. I get the small tabs and tuck them ito raw meat/hot dog pieces...500mg per day is a good place to start (though some people megadose with as much as 6000mg daily! Which will give most dogs the runs unless you work up to it.)
My SO is a people doctor & recommends vit C to anyone going through any sort of physical stress - from antibiotics, illness, etc. He also tells people not to smoke so make sure your dogs don't smoke, hehe.
Pumpkin is a good stool regulator, but won't do much to boost immune response. Probiotics a few times a week are a good idea too, keeps the intestinal flora flourishing. I'm quite a believer in a raw (BARF) diet for making a strong immune system, but if you do nothing else, try the vit C.