Are there any group insurance policies or individual policies which covers pre existing illness?

Some of the larger group policies will cover the employee with pre existing conditions, but it will depend on the contract each company has with the insurance company. So you will have to ask the company you are going to go to work for if thiers will or will not.

Most individual will not cover any serious policies but there are some that will if they accept you, unless they add an exception. But most will just not cover you if you have bad health.

Yes, there is group mediclaim policy which cover pre existing illness.