Bad breath is a symptom that is present by many people. Despite its unpleasantness it is sometimes caused by serious diseases.
Saliva helps to clean debris from the oral cavity and therefore oral dryness, regardless of its cause, is directly linked to an unpleasant odor. Drying of mouth can be caused by dehydration, smoking, certain immune diseases, etc (see relevant article).
Effect of food
Many types of foods and beverages affect smell of exhaled air. It is usual for aromatic food (like garlic) and for alcohol.
Poor oral hygiene and dental caries
This is a relatively common cause of bad breath. Mostly it is attributed to infrequent tooth brushing. If dental caries is present, the problems get worse. Odor occurs because of bacteria that are part of dental plaque and that produce many malodorous substances.
Infections of the oral cavity and anatomically close areas may be associated with bad breath. It may be a gum inflammation, purulent tonsillitis, sinusitis, etc. The same is true for infections of lower respiratory tract.
Return of gastric juice into esophagus and (and sometimes into pharynx or mouth) causes bad breath. Bad breath is present usually in the morning, because reflux is more common at night when lying in horizontal position.
Professionally it is known as gastroparesis. It has a number of causes; quite often occur as complication of long-lasting diabetes. Diabetes impairs innervation of digestive tract and worsens its motility leading to food accumulation in stomach. As therapy we usually administer drugs speeding up movement of GI tract (prokinetic agents).
Peptic ulcer complications
Chronic untreated peptic ulcers can sometimes spread (penetrate) throughout stomach wall into a nearby located part of colon and create a stomach-colon connection. This results in weight loss, because a large part of food is not absorbed in the small intestine and goes directly into colon through ulcer fistula. Foul odor from mouth is caused by large intestine (it is de facto odor of feces).
This condition may be characterized, among others, by typical odor of breath that is known as fetor hepaticus (“liver breath”). It is difficult to describe, I would compare it to an unpleasant sweet odor with a certain trace of fecal smell.
Untreated type 1 diabetes is associated with insulin deficiency and elevated blood sugar (glucose). When there is an excessive increase in blood sugar and disruption of internal environment, affected person looses consciousness and that is referred to as hyperglycemic coma. Lack of insulin in the body leads to formation of compounds known as ketone bodies. Keton bodies serve as a replacement source of energy instead of glucose. Aceton is one of these ketons and it is breathed out. Exhaled acetone causes a characteristic odor, which is unfortunately quite similar to alcohol smell. Unconscious diabetic in hyperglycemic coma lying in the street can be mistaken for a drunkard and left without help with possibly fatal consequences.
In this case, there is an accumulation of nitrogenous waste products in the body including ammonia and it causes characteristic odor of exhaled air.
Basic prevention is sufficient dental hygiene – regular tooth brushing, using dental floss after each meal. Regular fluid intake is advisable. Chewing gum also helps by promoting production of saliva and moistening oral cavity. Smoking is a big problem; heavy smokers will have always bad breath troubles. If bad breath is caused by other diseases, it is important to treat the underlying cause.