That Health Tracker Could Cost You

Using big data to improve health might seem like a great idea. The way private insurance works, though, it could end up making sick people -- or even those perceived as likely to become sick -- a lot poorer.

Suppose a company offers you an insurance discount and a free Fitbit if you agree to share your data and submit to a yearly physical. You're assured that the data will be used only in aggregate, never tied back to specific identities.