Medical data specifically. But I was also curious on your thoughts of all types of data. The worst that could happen is if the government forced you to reveal the data on specific people for whatever reason, or if the data was stolen for some reason.
The goal would be to potentially find common trends among humans to catch illnesses early on such as cancers and correct bad habits before they become worse or correcting bad diets that have harmful consequences such as people who are low on iron might have anemia or get early hair loss. It would also give an idea on how to help create people's ideal physical body.
I do know people are against sharing data in general for some reason. But I feel like such a thing could advance humanity if people just understood the human body more than they currently do.