how communicable are diseases?

Well, today we noticed one of the comets hanging out at the top of the
pond gulping air and being a little listless. Another downside of the
algae bloom, it's hard to see what's going on. We scooped him out and
took a look, and the poor little guy is pretty messed up. One of his
front fins is almost gone, he's got a worm hanging off his back, and a
nice fuzzy patch of fungus on one side. I can't tell if there's an
ulcer under the fugus, or if it's just fugus...

So, he's in a hospital tank full of our pond water. I've been testing
the water somewhat obsessively and it seems fine. Ammonia is 0,
Nitrates are around 2 and Nitrites are 0.25. We put in some medecines
for fin rot, external parasites, fugal infections, I don't know how
much that stuff helps but I figure it's worth a shot.

How likely is it that any of this stuff will catch onto the other fish?
They are both fine so far, the new guy that's only been in there a
couple of days (possibly the carrier?) has no signs of anything. I'll
obviously keep a close eye on them, but should I expect them to be
next?