I watched an episode of Air Crash Investigation, which depicts a plane catching fire due to it shipping many expired oxygen generators for planes. The narrator says that a fire in the cargo hold is very unlikely, as the cargo hold is vacuumed, to prevent fire; but however in this situation, the oxygen generators were actually generating oxygen so the vacuum was futile. I also know they carry pets in the cargo hold. So are the cargo holds pressurized, non-pressurized, or vacuumed? Or does it depend on the carrier, or aircraft?
The cargo hold is indeed vacuumed, using a vacuum cleaner, regularly. It is however NOT drawn vacuum (so having all air removed). Some cargo holds may be pressurised, some are not. Depends on the aircraft and the requirements of the cargo (life animals want a pressurised cabin of course, just to give one example).