Human snot is secreted by mucus glands distributed on the nasal mucosa. Under normal circumstances, about 500- 1000 ml is secreted every day, which is used to moisten the inhaled dry air, and can also take away foreign bodies such as dust. When everyone catches a cold, the main pathological changes are congestion and edema of nasal mucosa and increased secretion of mucus glands, so there are a lot of snot. Under normal circumstances, nasal mucus is colorless and transparent, but when a cold is accompanied by bacterial infection, the color of nasal mucus will change, and the specific color varies with different bacterial strains. Purulent nasal discharge is a typical manifestation of purulent lesions.