By definition, pre-existing conditions are those health conditions (enumerated as AIDS, diabetes, heart disease, asthma, high-blood pressure, and women) that you were treated for in the past or are currently receiving treatment for. Not all health insurance providers are created alike, and there are companies that completely cover pre-existing coverage after a waiting period and there are some companies that do NOT cover it at all.
In other words, having a pre-existing condition puts you more at risk of being uninsurable, although being relatively healthy does not guarantee you'll be accepted at any time in an affordable health insurance plan.
Under the Health Insurance Portability and Accountability Act of 1996 (HIPAA), an insurance company may or not cover pre-existing conditions. The HIPAA defines a pre-existing condition as a medically diagnosed mental or physical condition that you were treated for six months prior to your enrollment.