Players emerged in clusters, often grouped around an outstanding teacher.

ECONOMIST: 120 players from 35 countries have a ball

When it arrived in America it was detected within months thanks to the vigilance of the country's Centres for Disease Control and Prevention in noticing clusters of a previously rare skin cancer that is often a symptom of immune-system collapse.

ECONOMIST: Bird flu and public health

Indexing clusters investment assets in securities which represent past success, are widely owned, and often fully valued.

FORBES: Focus On Assets And Liabilities When Planning Your Portfolio