Six of those who contracted the H7N9 virus have died. All 21 cases have been reported in the eastern part of the country.
Health officials believe people are contracting the virus through direct contact with infected fowl and say there's no evidence the virus is spreading easily between people.
China's official Xinhua News Agency reported two new cases in Shanghai and one in east China's Anhui Province on Sunday, citing local authorities.
Shanghai has been ordered by the agriculture ministry to halt its live poultry trade and slaughter all fowl in markets where the virus has been found.
The capital cities of the neighboring provinces of Zhejiang and Jiangsu also have suspended sales of live poultry. Both provinces have reported H7N9 cases.
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