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.
More Nation & World Headlines
Cop indicted for murder in traffic-stop shooting Will ‘Happy Birthday’ song be free to use soon? Debris linked to missing Malaysian 777 Export-Import Bank excluded from highway bill 777 departs Aleutians after emergency landing House approves highway funding bill Taliban leader Mullah Omar dead, Afghans say Administration wants to give prisoners access to Pell grants
Our new comment system is not supported in IE 7. Please upgrade your browser here.