A pre-existing condition is usually defined as any medical condition, whether illness or an injury, that required treatment prior to the person taking out health insurance. The specific terms vary among insurance providers -- some include conditions known about only three months before the effective date of coverage, while others exclude coverage of any pre-existing condition for as long as a year after the effective date of coverage. While some providers include only conditions that required treatment, others include any condition the insured knew about, even if it did not require treatment.