Pre-existing diseases are those medical conditions which existed even before purchasing the insurance. High blood pressure, diabetes, asthma, cancer, etc. Are some of the common pre-existing diseases which are to be declared at the time of purchasing a policy. Different insurers have different policies and while most policies have a waiting period of 48 months, there are some with a lock in period of just 24 months. Customers with pre-existing conditions are advised to compare and shop extensively to find a more suitable policy.