A pre-existing disease refers to any illness, injury, or medical condition that existed prior to the date of the commencement of the policy. This can include Diabetes, Hypertension or even Cancer.
Treatments for pre-existing diseases are covered and are payable by the insurance company, but only after the policy has been effective for a certain amount of time. Waiting periods for pre existing diseases typically range from two to four years, depending upon the insurance company and the type of pre-existing disease.
For senior citizens though, there are plans that cover pre existing conditions after a waiting period between one to two years as well.
To know more see Waiting period in a Health Insurance Policy