Causes of bad breath

Cause: Oral Diseases and Infections

Periodontal disease is directly related to improper or neglected brushing and flossing. One major sign of this potentially irreversible oral disease is halitosis. The accumulation of plaque, bacteria, and decomposing food particles contribute to bad breath as they destroy the delicate tissue that surrounds our teeth.

The same bacteria that cause gum disease, tooth decay, and abscessed teeth are also responsible for halitosis.

Cause: Dry Mouth

Xerostomia is a condition that causes a decrease in the production of saliva, resulting in a dry mouth. Several factors cause xerostomia, some of which may need to be treated by your doctor.

Saliva is necessary to provide lubrication the mouth to allow for proper chewing and swallowing. Saliva naturally cleanses the mouth and helps prevent cavities. If you are experiencing dry mouth, bad breath may occur because the food particles remain trapped in the mouth to rot and cause the unpleasant smell when you exhale.

and there's more

https://www.verywellhealth.com/what-causes-bad-breath-1059324