My take....

The animal has a heavy parasite load, and the respiratory infection is secondary. Basically, bacteria that are commonly present have been allowed to multiply to an unsafe level because his immune system was already jeopardized.

This is what's commonly referred to as an opportunistic infection. Not a result of being exposed to the bacteria itself, but a result of the immune system not being able to fight off what would otherwise be a normal level of that bacteria.

I'm no vet, but I know what I would do.

1. "De-worm" the snake multiple times with Flagyl and Panacur...just to make sure parasites are a non-issue.

2. The drug you're using (fortaz is the common name) works much better for RI when it's used in conjunction with something like Baytril or Amakacin, so I would ask the vet about getting a bottle of Baytril if you can.
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Not to throw stones (we've all been suckered in by a seemingly good deal at a show), but this is a perfect example of why it's best to buy from a breeder you know. Whatever may have been saved on your spider by getting it at a show is hardly worth all the time and energy you've put into this....and that's not to mention that you have a snake that is trying to recover, rather than growing up to be a breeder. It took me a long time to get that, so don't think I'm grilling you...just pointing out that the truly good deals sometimes cost a few bucks more up front.