HealthVault is an online service from Microsoft to track, record and store health/medical related information. The Windows Phone app, HealthVault, is a mobile client for this service to let you have access to all your health and fitness information while on the go. The HealthVault service lets you document your medical information, list emergency contacts, track chronic conditions, and upload medical documents (via the online portal). Online access will also recommend health and fitness apps for your Windows Phone.
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Windows Phone App Review: Microsoft HealthVault