Bad breath in cats, also known as halitosis, can be caused by a build-up of odour-producing bacteria in your cat's mouth. This can be due to dental or gum disease but constant bad breath can also be a sign of more severe medical problems like abnormalities in the mouth or respiratory system.
1 Additional Answer
Your cat could have bad breath because they are eating wet food, canned food, or table food. You should feed your cat dry food because it is better for them.