Until this week, no cases of human-to-human transmission of the deadly bird flu virus that broke out in China this year had been reported.
But now, researchers say they believe a woman in the eastern Chinese city of Wuxi probably caught the H7N9 virus from her father.
The woman, 32, most likely became infected when she was looking after her father at his bedside in the hospital in March, according to a study published Tuesday in the British Medical Journal. Both patients eventually died.
The woman had no known exposure to poultry, the study said.
"Our findings reinforced that the novel virus possesses the potential for pandemic spread," the study's authors wrote.
But in the case they studied, they said "the transmissibility was limited and non-sustainable."
In April, the World Health Organization warned the H7N9 virus was "one of the most lethal" that doctors and medical investigators had faced in recent years.
The virus has caused 132 human infections resulting in 43 deaths since February, Chinese state media reported last month.
The rate of infections seen earlier this year has dropped off dramatically.