The mouth itself does not cause bad breath, but diseases in the oral cavity can cause bad breath. In addition to systemic diseases of the digestive and respiratory systems, most diseases that cause bad breath probably originate in the oral cavity. More common are periodontal diseases, such as chronic periodontitis, necrotizing ulcerative gingivitis and post-extraction infections, which may cause bad breath. Simply put, bad breath mainly refers to the odor of the mouth, and once there is an infected lesion in the mouth, it means that there is a large amount of pathogenic bacteria in the mouth. For the oral cavity, a large part of the pathogenic bacteria is anaerobic bacteria, anaerobic bacteria there is a kind of bacteria called aerobic podococcus, which itself will break down the substances in the mouth, thus producing gas containing sulfur, this gas will contain a distinct odor, which forms bad breath. Likewise, inflammation in areas including the nasal cavity and maxillary sinuses may also produce an odor in the mouth through the oral cavity. For these diseases, it is necessary to find the cause and treat the cause.