It sounds like the vet treated it wrong anyhow, whatever it was. Most of us here know that you never lower the temperatures on a sick snake--you elevate them slightly. Lowering the temps just to start antibiotics is going to lower the snake's immune system, and with a virus, all the antibiotics are doing is combating secondary infections. So that was exactly the wrong treatment for a viral outbreak.
It could have been IBD, but with the symptoms described, I would put a first guess on OMPV, with improper treatment explaining why there were no survivors at all. But it could also have been one of the other half-dozen deadly bugs (rhabdovirus or reovirus, etc).
Whatever it was, however, was probably not airborne, or the boas would have caught it too.