How a decades-old database became a hugely profitable dossier on the health of 270 million Americans

To most Americans, the name MarketScan means nothing. But most Americans mean everything to MarketScan.

As a repository of sensitive patient information, the company's databases churn silently behind the scenes of their medical care, scooping up their most guarded secrets: the diseases they have, the drugs they're taking, the places their bodies are broken that they haven't told anyone but their doctor. The family of databases that make up MarketScan now include the records of a stunning 270 million Americans, or 82% of the population.

The vast reach of MarketScan, and its immense value, is unmistakable. Last month, a private equity firm announced that it would pay $1 billion to buy the databases from IBM. It was by far the most valuable asset left for IBM as the technology behemoth cast off its foundering Watson Health business.

Imagine how easy it would be for companies to hire only people in tip-top health, and disregard anyone with even the smallest of preexisting conditions. This data is hugely valuable to just about anyone.