The phrase "fatal illness" refers to a sickness or disease that can cause death. It means that the illness is very serious and has the potential to be deadly. Full definition
Regardless of how well their bodies can handle changes in weather, they are all vulnerable to being hit by cars, being attacked by other animals, or contracting potentially fatal illnesses. (cathealth.com)
Others are the result of fatal illnesses, such as cancer. (chai-online.org)
This plan provides a working woman with triple coverage: it includes a life cover, savings in the long term, and includes a clause for fatal illnesses. (policybazaar.com)