The honest answer has two halves, and most sites only tell you the half that suits them.
On an ACA Marketplace plan the answer is simple: pre-existing conditions are covered, always, by law. Your health history cannot be used to raise your rate or deny you a policy. That protection is the single strongest argument for the Marketplace.
On private underwritten plans
Here it depends on the condition, how well it is managed, how long ago it was diagnosed, and which carrier you apply to. Four outcomes are possible for any given condition: covered at standard rates, covered at a higher rate, covered after a waiting period, or excluded by rider.
Well-managed, stable conditions are routinely accepted. Something diagnosed recently and still being investigated is a much harder application, and nobody can tell you the answer from a web page.
What to do about it
Tell a licensed advisor your actual history — all of it — before anything is submitted. An advisor who knows the carriers can tell you which ones treat your condition well and which will not take it, and can also tell you when the honest recommendation is to stay on a guaranteed-issue plan instead.
If a plan is being sold to you without anyone asking a single health question, be careful. That usually means it is not major-medical insurance at all.
