In the context of healthcare in the United States, a pre-existing condition is a medical condition that started before a person's health insurance went into effect. A pre-existing condition can be something as common as high blood pressure or allergies, or as serious as cancer, type 2 diabetes, or asthmaâ€chronic health problems that affect a large portion of the population. Americans with preexisting conditions need health insurance. This book looks at the legislation concerning preexisting conditions.