The Chinese capital reported on Saturday its first case of a new strain of bird flu, state news agency Xinhua said, the first time it has been found in a human outside of eastern China.
The seven-year-old girl is in a stable condition in a Beijing hospital, the report said.
Two people who have had close contact with the child have shown no signs of being infected so far, Xinhua added.
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The girl's parents are engaged in the live poultry trade, it said.
The girl developed a fever, cough, sore throat and headache - all flu symptoms - on Thursday morning before she was brought to the hospital, according to the report.
She was given the drug Tamiflu, received intravenous drips on Thursday night, and was transferred to an intensive care unit when her condition worsened, Xinhua said.
Her breathing has improved and temperature dropped, it said.
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The website of China's state radio showed a photo of the girl lying in bed, wearing a large blue face mask and with a stuffed doll next to her.
So far 11 people have died of the H7N9 bird flu strain since it was confirmed in humans for the first time last month, with 44 infections in all having been reported to date.
Shanghai and the eastern provinces of Zhejiang, Jiangsu and Anhui had been the only confirmed locations of infection until the Beijing case.
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The source of infection remains unknown, though the virus has been found in some birds in poultry markets that remain the focus of investigations by China and the U.N. Food and Agriculture Organization.
The new virus has caused severe illness in most of the people affected, leading to fears that if it becomes easily transmissible, it could cause a deadly influenza pandemic, though there has been no indication of that happening.
In a bid to calm public jitters over the virus, Chinese authorities have detained a dozen people for spreading rumours about the spread of bird flu.