Health insurance companies
How insurers make sure you're not a bad risk
Health insurance companies are just like any other insurer. Their prices are based on risk, and to keep premiums as low as possible they won't cover high-risk patients. Typically they won't cover you for injuries resulting from dangerous hobbies, and they won't pay for treatment for ailments you already have.
Just like car insurance companies and home insurance companies, health insurance companies have limits: in much the same way that a car insurance firm won't cover a 17-year-old driving a ridiculously fast sports car, health insurance firms won't cover certain risks. You'll find that injuries from dangerous hobbies won't be covered, and treatment resulting from HIV/AIDS or drug abuse won't be covered.
Another way in which health insurance companies minimise risks is to exclude "pre-existing conditions". These are ailments that you already have when you take out your policy, so for example if you're undergoing treatment for back problems then your policy won't start paying for that treatment when you start making your monthly payments. Some insurers will eventually cover these conditions, though: if you haven't received treatment for a period of two years, then health insurance companies will usually pay for any treatment should the condition reoccur.