The causes of oral odor can be divided into two aspects: ① oral diseases caused by: common for gingivitis or periodontitis, mostly due to poor oral hygiene, residual food in the mouth, more tartar, resulting in gum inflammation; periodontal pockets pus, necrotic tissue and bacterial metabolites lead to bad breath; in addition, such as necrotizing gingivostomatitis, post-extraction infection and some other necrotizing inflammatory diseases can also cause bad breath. ② Systemic diseases: such as the smell of rotten apples in the mouth of diabetic patients; some digestive and nasopharyngeal diseases can also cause bad taste in the mouth.