Some were found within family clusters, though WHO experts say it isn't clear whether those cases were caused by a common exposure to poultry or due to limited person-to-person transmission.

WSJ: Bird Flu Found Outside China's Mainland

Some cases were found within family clusters, though WHO experts say it isn't clear whether those cases were caused by a common exposure to poultry or due to limited person-to-person transmission.

WSJ: Bird Flu Found Outside China's Mainland

It then used this information to construct a family tree that placed them into clusters.

ECONOMIST: Computers and language