How does the insurance company decide whether a disease was a pre-existing one or not?
When you buy a health insurance policy, you need to provide details of any illnesses you have suffered/treatments you have undergone during your lifetime. The insurance company will then refer all your 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 from before buying the health insurance policy. Insurance is a contract based on good faith and any wilful non-disclosure of facts might lead to problems in the future.