This depends on the insurance company with whom the policy is taken, as well as the cover opted for. Certain illnesses, however, are excluded by most critical illness cover and these include HIV and AIDS, self-inflicted injuries, injuries related to war and civil unrest, drug/substance abuse, criminal acts, dangerous sports and hobbies, etc. Exclusions are also applicable if the policyholder violates the terms of policy by ignoring medical treatment. For pre-existing conditions, the policyholder must give complete disclosure to get a clear idea of what is included and what is not. For a full list, compare insurance quotes online.