BOSTON —

The key to living a long and happy life hasn't been found yet.

But some researchers believe it could be hiding inside the Harvard Chan School of Public Health.

That's where nearly 50 years of data from the Nurses' Health Study is stored.

"We've collected blood samples, urine samples, toenail samples," said Dr. Walter Willett, a professor of epidemiology and nutrition. "And we've collected many of these samples repeatedly over time."

The number catalogued at the school's biorepository is in the millions.

Since 1976, more than 280,000 nurses of different ages and backgrounds have donated their own biological specimens and provided detailed information about their health, lifestyle, and medications for researchers like Willett to study.

"We have dozens of big nitrogen freeze

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