A pre-existing condition is a health condition or illness that existed before the start of a new health insurance policy. Prior to the Affordable Care Act (ACA), insurance companies could deny coverage or charge higher premiums to individuals with pre-existing conditions. However, the ACA prohibits insurance companies from denying coverage or charging higher premiums based on pre-existing conditions. Under the ACA, all best health insurance plans must cover essential health benefits and cannot discriminate based on health status. This provision has allowed millions of Americans with pre-existing conditions to obtain affordable health insurance coverage. It's important to note that the future of pre-existing condition protections is uncertain, as ongoing legal and political challenges could impact these protections in the future.