Bad breath can be caused by bacteria and types of food left in the mouth!
Gum disease can lead to bad breath.
What can help bad breath?
Clean your tongue (especially the back).
Tongue brushing can help bad breath by 70%!
Different dental appliances (like dentures and retainers) also need to be cleaned as well.
Dry mouth can lead to bad breath - so addressing this is important.
Normal dental visits can keep your gums and teeth clean.
There's other systemic (body) reasons that may cause bad breath, a physician can be helpful in discovering these.
More info:
https://www.mouthhealthy.org/all-topics-a-z/bad-breath