Denial is the first stage in the DABDA theory of coping with death. Most people will enter this stage first after receiving a terminal diagnosis.
We all want to believe that nothing bad can happen to us. Subconsciously, we might even believe we are immortal. When a person is given the diagnosis of a terminal illness, it's natural to enter a stage of denial and isolation. He may flat-out disbelieve what the doctor is telling him and seek out second and third opinions. He may demand a new set of tests, believing the results of the first ones to be false. Some people may even isolate themselves from their doctors and refuse to undergo any further medical treatment for a time.
During this stage, one might also isolate himself from his family and friends to avoid discussions about his illness. He may believe on some level that by not acknowledging the diagnosis it will cease to exist.
This stage of denial is usually short-lived. Soon after entering it, many begin to accept their diagnosis as reality. The patient may come out of isolation and resume medical treatment.
Some people, however, will use denial as a coping mechanism long into their illness and even to their death. Extended denial isn't always a bad thing; it doesn't always bring increased distress. Sometimes we mistakenly believe that people need to find a way to accept their death to be able to die peacefully. Those of us who have seen people maintain denial until the end know this isn't always true.
Those who successfully move through denial will usually enter a new stage of coping -- Anger.