How do health insurance premiums work?

Are premiums raised every time you get sick like premiums are raised every time you get in an auto-accident? If you get an existing condition WHILE being covered under an individual plan, do they raise premiums by a lot?
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No, they don't surcharge you for putting in claims or getting sick. When you first apply, they put you in a "category". You stay in that wellness category, and your rates only go up when you get to a new AGE category. They can't downgrade your wellness category, and they can't surcharge you or cancel you because of claims. It's not at ALL like car insurance.
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