Any incident of diagnosis and treatment of disease is called pre-existing. Any incident of hospitalization or operation should be informed to the insurance company. Any incident where you have availed the health insurance benefit through personal or group insurance needs to be intimated too. You should give all information honestly and let underwriters decide whether to accept the proposal or not.
Insurance Company classifies the diseases which have happened in the last 48 months as pre-existing diseases. These diseases can be subject to exclusion as per the terms and conditions of the insurance company.