A pre-existing condition is a health or medical problem an individual has before applying to be accepted into a health insurance plan.
Most insurance plans will deny insurance to someone who already has a track record of health challenges because they don't want to pay for known problems. Even if someone fails to mention a previous health problem on an application for health insurance, the insurer may uncover previous problems through the Medical Information Bureau.
Some insurers will conditionally accept a new customer by providing an exclusion period. You may be accepted, and pay for the insurance, but no care related to your pre-existing condition will be covered for a period of time which may range from six months to 18 months, depending on the policy and state regulations.
Pre-existing conditions are considered part of the conversation about healthcare reform since people who have health problems are excluded from access to a payment system.

