Health officials in China are scrambling to uncover how multiple members of three families in Shanghai and a young boy and girl from neighboring homes in Beijing became infected with a new strain of bird flu.
The H7N9 virus, which has killed 17 and sickened at least 82 people since March, is thought to pass from birds to humans. But its spread within families and neighborhoods has flamed fears about possible human-to-human transmission.
More than 1,000 close contacts of those infected are under close watch for signs of the disease, according to the World Health Organization.
"We don't think there's sustained human-to-human transmission, because the only instances where there might have been human-to-human transmission are between two close family members," said WHO spokesman Gregory Hartl, explaining that the family members and neighboring children may have been exposed to the same infected bird.
"The main thing now is to figure out how this virus spreads and where it lives. Until then, we're shooting in the dark," he said.
To complicate matters further, many of those among the 82 infected reported having no contact with birds in the time leading up to their illness, according to Hartl.
"That makes it important to investigate all possible routes of exposure," he said, adding that other animals could be serving as virus-spreading middlemen. "That there are 82 cases now doesn't change the fact that they're still all environmental or animal-to-human transmission. We are not seeing human-to-human transmission."
ABC News' chief health and medical editor Dr. Richard Besser, who served as acting director for the U.S. Centers for Disease Control and Prevention during the 2009 swine flu outbreak, called this a "critical point" in the investigation.
"While it's possible that this is being transmitted exclusively from birds to people, there hasn't been enough information released to conclude that," he said. "If appropriate control measures are to be implemented, it's critical to know more about transmission."
Besser said there are techniques to identify the source of the spread and learn if certain behaviors increase the risk of infection.
"Hopefully those studies are being done, but there hasn't been enough information shared to know that," he said.
Hartl said an international team of experts was just arriving in China to support Chinese health officials in their investigation. The group is expected to issue a report next week.
The CDC is sending two epidemiologists and one laboratory scientist to aid in the investigation, according to Dr. Joe Bresee, lead epidemiologist in the agency's influenza division.
"The working assumption is that the infection is due to exposure to infected poultry. However, I would not close any doors," Bresee said. "It is too early to tell for sure."