what caused RI in my ball python

i have a pastel 100% pied 800g was doing fine for the 5 months i had him. today i went to clean the cage and heard a wheezing sound. so i checked up on him he had bubbles coming out of his mouth and was yawning. his bedding (aspen laboratory grade shavings) stuck to the sides of his mouth. the temps range 91 during the day and 78 at night. humidity is at 40%. he ate 3 days ago. i change the water every other day. I go to the vet 2morro and i have quarantined him. does anyone know why this happened? how long ri take to be deadly. im gonna be up all night hopefully he doesn't die. it would really put the halt in my bumblebee pied project.

Hard to tell. Some people have told me never to let the cool drop below 80, but all last season I dropped it to 78 for my breeders and had no problem. Low humidity and high humidity can supposedly cause it. Any drafts? Near a window? Enclosure or rack? Is he by chance on the bottom of the rack? Are those the temps on the thermostat or actual readings in the tub?

I don't have much experience, but I had one snake shipped to me with an RI and I seen one that my brother ordered with an RI and niether had bubbles or yawaning yet. The vet said they get that a little further done the road. I've also heard that some snakes are just prone to getting RI's a lot.

I just re-read your post. If you let your hot spot drop to 78 at night, how low is your cold spot? That may be your problem. My night time drop is only 1.5-3 degrees depending on the time of the year.

Since the snake was healthy when you received it, and didn't show any signs of illness for several months...in all likelihood it was something husbandry related. There isn't enough information posted to make a determination of WHY your snake got sick - if you describe how the snake is housed, what you use for substrate, how the heat is provided (and how you measure temps); we might be able to help a bit more.
As Nick mentioned, low humidity is often a factor....as would be poor ventilation and unclean conditions.

i took him to the vet and they gave me baytril injections. but only for a month. does anyone know where i can buy a vial of it to save me trips and money to the vet?

Legally u cant buy it here in the US without a vet Prescription.

For a month????
How often were you told to give the injections?

every other day only 2 cc. He gave me 7 doses. i was just wondering cause it cost me 90 bucks for the injections. if i could buy it i would. the vet charged me another 40 because the vet gave him a shot of baytril himself. but the shots are only 12 a peice. he told me if it still hasnt gone away to come back. however that would be another 180 out of my pocket. i i know i sound like im cheap but.
i know everything was right on my end. i clean cages 3 times a week. 2 spot cleans and a full cleaning. (2 tablespoons of bleach the rest water. in a spray bottle.) All i know is i got a sick snake and it sucks.

Stress?

7 doses every other day is only 2 weeks, not a month. What do you use to check your temps? how are you housing your snake? How did the vet test to see if it was an RI? Also what kind was it? Bactirial? Viral? Those factors may be able to help figure out what caused it. What is the rooms' ambient temp? Please give all the info you can.