How does the insurance company decide whether a disease was a pre-existing one or not?
While filling up the proposal form for insurance you need to provide details of the illnesses you have suffered during your lifetime. At the time of insurance, you should be aware of whether you have any disease and whether you are undergoing any treatment. The insurers refer to such health issues to their medical panel to differentiate between pre-existing and newly contracted illnesses.
Note: It is important to disclose any disease you might be suffering with before buying the health insurance policy. Insurance is a contract based on good faith and any willful non-disclosure of facts might lead to problems in the future.