Bad breath, also known as halitosis, is mainly caused by a bacteria that is allegedly anaerobic. The bacteria lives off of dental plaque on the teeth and gum line. Plaque tends to build up in the places in the oral cavity that lack oxygen the most.
Certain foods can cause halitosis more than others, like onion, garlic, and other spices. The smell caused by anaerobic bacteria, however, lasts much longer and is stronger than the bad breath smell caused by certain herbs. These bacteria also get nourishment from certain foods we eat, especially meat, milk, and fish. That is why proper oral hygiene is so important!
Xerostamia, more commonly known as dry mouth, is a condition that occurs when there is a lack of saliva in the mouth. Saliva actually has a very important role in keeping the mouth clean and removing food particles. Without these actions, one is much more likely to have bad breath. The salivary glands tend to stop working when one is asleep, which is why morning breath is so common.
Certain medical conditions are known for causing halitosis. Sinus problems and discharges, as well as other biological secretions can come out of the oral cavity, thus contributing to the smell of malodor. Tonsillitis, chronic lung infections like bronchitis, gastritis and other throat infections can cause bad breath. Also, if one has diabetes and does not control it, he or she may have a fruity breath smell, whereas liver failure can cause a fishy smell.
Lastly, if you have any concerns about your medical condition, you should go see a licensed physician.