Orphans -- Phil? Anybody?

I've got two 1-week-old orphans to raise on a bottle. I've done it
many times, and have, so far, not lost any.

But, every single time I've raised orphans, about the time they're 8
weeks & older, the are *extremely* susceptible to URI's. I know the
orphan babies are not getting natural immunity from mother's milk. I
always get their vaccinations the moment they are old enough. That
doesn't seem to completely keep them from getting "kitty colds".

They're on KMR. I was wondering if there is any supplement, vitamin
drops, anything that will help boost their immunity.

Thanks!

Sherry