July 28th. According to relevant data, the epidemic of Yangzhou 202 1 started on July 28th, and the source of infection was a 64-year-old woman named Mao Mouning in Jiangning, Nanjing. She left the place where control measures had been taken during the quarantine period, took a bus to Yangzhou on July 28th, and stayed at her 70-year-old sister's house, after which her sister was diagnosed.