Researchers have released information that the H7N9 bird flu discovered in China can transmit between humans.

This is the first conclusive proof of human-to-human transmission. A 32-year-old woman has become infected with the virus after caring for her father. Both of them died from multiple organ failure.

The scientists say that by the end of June China has confirmed 133 cases of H7N9 and 43 deaths.

The scientists are emphasizing that just because they have confirmed a human-to-human transmission of the virus is does not mean that the virus can easily pass between humans.

“It would be a worry if we start to see longer chains of transmission between people, when one person infects someone else, who in turn infects more people, and so on,” Dr. James Rudge of the London School of Hygiene and Tropical Medicine told the BBC. “And particularly if each infected case goes on to infect, on average, more than one other person, this would be a strong warning sign that we might be in the early stages of an epidemic.”