> The company, owned by the massive UnitedHealth Group, has collected the medical diagnoses, tests, prescriptions, costs and socioeconomic data of 150 million Americans going back to 1993<p>I think they just use their point of sales (primary care, etc) to collect data from the client and use it against the client. I know because I used to be in a very similar line of business.<p>It is easy because clients give you a permanent unique id for payment purposes (ssn), or other unique id (phone numbers) that varies with time (now less with people porting their number). Of course, the point of sale has a list of previous items, even from competitors (medical history) but that has become harder to use, due to laws. Still, most places ask for "emergency contact", which you can use to build a social network. Sick people cluster together. I don't know why, it just happens, and it is a good workaround.<p>Of course, you need enough data, but it is then a matter of scale (if you have 75% of the market, you have seen everyone in a county at least once) and trade (buy the same data from your competitors).<p>Personally, due to experience, I prefer to forego insurance and get my healthcare abroad. Better prices than paying deductibles, better services. But I can not recommend that for everyone.<p>Still this is a dirty business, and I strongly recommend to adopt basic opsec precautions if you get healthcare in the US: never give your ssn, give a phone number that is not used for anything else even better if it is prepaid so not linked to a ssn, never ever give an emergency contact, only list medical conditions that will not cause you legal issues.