What do you do when they get flu?

[Bogertophis]

how do they get the respiratory infections? ...

... a respiratory infection (RI) usually comes from a combination of poor humidity, temperatures, and stagnant air...

And from dirty cages or air-borne pathogens. Just like humans don't "catch colds & flu from cold air", snakes don't acquire an RI (respiratory infection) from just "iffy" temperatures & humidity.

In the case of snakes- for those that brumate (snooze for the winter at lower temperatures), they ARE at risk from any pathogens they might be harboring prior to brumation, even IF they appear to be perfectly healthy. That's why brumation is best only done when necessary for breeding (cooling promotes fertility), because it's not without risks- now & then, snakes wake up sick after their winter sleep, & more rarely, a few may not wake up at all. So just remember that really cool temperatures shuts off a snake's immune system. I have brumated snakes in the past, when I bred some- I never had any problems, but it's always a possibility.

The warmth & humidity in a snake's cage (especially BPs because they require warmer temperatures & because the air-flow is usually restricted) promotes the survival of "germs"- various pathogens (bacteria & viruses) that also hang around and multiply in enclosures that aren't kept clean enough. This includes the substrate, the water bowl, all furnishings, & the physical enclosure itself. If the enclosure is made of wood, for example, that wood can absorb & then continue to supply pathogens. Pathogens may also be shared from other animals etc, via the air in the room. (This is why we quarantine new pets.)

Just like us, the more you can reduce the number of pathogens, the less likely that a healthy & well-cared for snake will get sick. But when they're kept "24-7" in a dirty cage, it wears down their immune system. Their body may be able to fend off some germs, but not a bunch of them that are "permanent party" in their homes. BTW (Reptilsoul), I'm not suggesting your cages actually are dirty- that's just one possibility. Also, snakes that are stressed (excessive handling, especially when newly acquired) may have less efficient functioning of their immune system.

Don't forget that when snakes are shipped to us (owned or bred by someone else first) they bring all that previous exposure with them, so if they aren't given quality time to "settle in" (so their immune system is up & running better) they're more likely to get sick, even IF you've kept them clean. Just like us, stress matters.