I've kept snakes for years, mostly various oddball colubrids and boas, and have used setups ranging from humid with limited venting, to airy and dry and never had a respiratory infection in any of them. I've also had both steady temps and temp drops of nearly 10f and honestly never noticed any health changes because of it. I've come to believe that sometimes even under ideal conditions that certain species will occasionally just get sick, much like humans do. I'm thinking your setup was probably fine in this case, and that you were just unlucky enough to have the wrong bacteria hitch a ride somewhere at just the right time.