Private health insurers retroactively dropping coverage for sick - what exactly are we insuring?

If you're going to need Health Care anytime soon, make sure you don't "get" any of these more "costly" conditions:

breast cancer, lymphoma, pregnancy, or high blood pressure.

If you do, it will put you on the Insurer's Radar, for possible Policy Cancellation...

What are we insuring exactly, when private health insurers retroactively drop health care coverage when we get sick? It clearly isn't our health.