It could be as simple as food that they ate to poor oral care or an underlying condition. The following gives more detail on what could cause bad breath:
Breath odor is the scent of the air you breathe out of your mouth. Unpleasant, distinctive, or offensive breath odor is commonly called bad breath.
Bad breath; Halitosis
Some disorders will produce specific, characteristic odors to the breath.
A fruity odor to the breath occurs as the body attempts to get rid of excess acetone through the breathing. This is a sign of ketoacidosis, which may occur in diabetes. It is a potentially life-threatening condition.
Breath that smells like feces can occur with prolonged vomiting, especially when there is a bowel obstruction. It may also occur temporarily if a person has a tube placed through the nose or mouth to the stomach to drain the stomach contents (nasogastric tube) in place.
The breath may have an ammonia-like odor (also described as urine-like or "fishy") in people with chronic kidney failure.
If previously normal breath turns into halitosis, causes could include:
Triamterene and inhaled anesthetics
Insulin - injection
Food or beverages consumed (such as cabbage, garlic, raw onions, or coffee)
Foreign body in the nose (usually in children)
Often (but not always) there is a white, yellowish, or bloody discharge from one nostril
Gum disease (gingivitis, gingivostomatitis)
Poor dental hygiene
Vitamin supplements (especially in large doses)
Diseases that may be associated with breath odor (not presented in order of likelihood -- some are extremely unlikely):
Acute necrotizing ulcerative gingivitis
Acute necrotizing ulcerative mucositis
Acute renal failure
Bowel obstruction (can cause breath to smell like feces)
Chronic renal failure (can cause breath to smell like ammonia)
Diabetes (fruity or sweet chemical smell with ketoacidosis)
Gastrojejunocolic fistula (fruity-smelling breath)
Ozena, or atrophic rhinitis
Use proper dental hygiene (especially flossing), and remember that mouthwashes are not effective in treating the underlying problem.
Fresh parsley or a strong mint are often effective ways to fight temporary bad breath. Avoid smoking. Otherwise, follow prescribed therapy to treat the underlying cause.
Call your health care provider if
Breath odor persists and there is not an obvious cause (such as smoking or eating odor-causing foods).
You have breath odor and signs of a respiratory infection, such as fever, cough, or face pain with discharge from the nose
What to expect at your health care provider's office
Your doctor will take a medical history and perform a physical examination.
You may be asked the following medical history questions:
Is there a specific odor?
Is there a fishy smell?
Does the breath smell like ammonia or urine?
Does the breath smell like fruit or is there a sweet-chemical smell?
Does the breath smell like feces?
Does the breath smell like alcohol?
Have you recently eaten a spicy meal, garlic, cabbage, or other "odorous" food?
Do you take vitamin supplements?
Do you smoke?
Does good oral hygiene improve the odor?
What home care measures have you tried? How effective are they?
Is there a recent sore throat, sinus infection, tooth abscess, or other illness?
What other symptoms do you have?
The physical examination will include a thorough examination of the mouth and the nose. A throat culture may be taken if you have a sore throat or mouth sores.