Gastroenterologist Aigul Ishmurzina listed the main causes of bad breath.
This problem is relevant not only for adults, but also for children. Most often, it occurs due to problems with teeth, certain foods. Sometimes diseases of the digestive system become the cause of an unpleasant odor.
“The reason for the appearance of an unpleasant odor can be a certain kind of diet, for example, eating a large amount of protein foods, carbohydrates, fruits, legumes. The solution in this case is to change the diet to a more balanced one,” says the expert.
As for other causes, in particular concomitant diseases, it is necessary to determine where the problem comes from: the oral cavity, nasopharynx or teeth.
Check the quality of brushing your teeth when it comes to a child. Go to the dentist for a checkup.
Having ruled out the first cause, let's deal with the upper respiratory tract. Perhaps there is chronic rhinitis, tonsillitis, adenoids, sinusitis. With these pathologies, we often encounter the growth of pathogenic bacteria. They become the cause of an unpleasant odor.
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An unhealthy gut microbiome, parasitic and fungal diseases can also be the cause of odor.
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If none of the pathologies is confirmed, contact the specialists: dentist, ENT specialist, gastroenterologist, pediatrician. 
Aigul Ishmurzina Health Gastroenterologist