Some US hospitals are buying personal data to warn people they might get sick.

I see no harm in this particular practice. If it seems creepy, that's because it is the consequence of something that really is creepy: the collection of all that data about people. That's what's really wrong.

I would not mind if my own computer kept a log of my purchases. I might find it useful to run a free program to look through them for things to warn me about. I don't want a data broker to have that data, which is why I always buy anonymously, paying cash.