Some villages have ailments related to heavy metal poisoning, while others have suspicious clusters of cancer.

WSJ: Review & Outlook: What's in China's Water?

The maths tells us that in respect of probability, people are much like tosses of a coin: we will see occasional clusters, even of cancer.

BBC: NEWS | Programmes | More Or Less | Patterns in Randomness

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