Officials said that a technician reused a tube used to treat an individual with HIV on other patients.
Provincial authorities described it as a "severe violation of procedure".
Five people had been sacked at Hangzhou's Zhejiang Provincial Hospital of Chinese Medicine, they said.
Provincial health officials said they were informed of the situation on 26 January.
But in a statement (in Chinese) they gave no information on how many other patients might have been exposed, what they were being treated for or when the infections occurred.
Those affected would receive treatment and compensation, the brief statement said.
Low safety standards and insufficient regulation helped spread HIV/Aids in China two decades ago, and the news of the hospital incident sparked shock and criticism from social media users.
"If a provincial-level hospital doesn't follow protocols, who can we trust as average citizens?" wrote one person on Weibo, China's version of Twitter.
"This case is exposed, but what about cases that we don't know? There must have been many more!" wrote another.


