I had one of my male cats neutered at the local Humane Society in
Redding, CA and they sent him home with a terrible Respiratory Illness
that spread over night to my other two cats.

Haven Humane Society refused to treat my cat[s] and I panicked when they
went into dark places to [die, I thought].

I ended up getting a pound of Tetracycline for farm animals for
something like $5-6 bucks that was water soluble. A little bit goes a
long way. In 3 days all my cats were back to normal. To play it safe I
treated the water for ten (10) days. After the fifth (5th) day I cut the
dose in half.

I think a gram of tetracycline treats 1 gallon of water. I eventually
just poured half the original dose into an empty gallon jug and
topped-off each jug. Tetracycline is a broad spectrum antibiotic. Kind
of what that other guy suggested.

Change the water every day whether its treated or not. I clean or scoop
my litter boxes twice daily also.

If you can't get the meds in the nose like the other post suggested;
just apply a little on their front paws. They'll lick it off and at the
least ... get it into their system. I apply vitamin paste in this
fashion since they fight me otherwise or ignore it on their bowls.