Based on everything up to chapter 18, what is Joseph Heller trying to say about death?

The war has suddenly made them realize how fragile life is and how close people always are to the brink of death. The inability to accept that sickness and death from disease is a natural process of life results in people’s insane behavior. Doctors are not immune from such fears either, for Daneeka is very susceptible to Yossarian's sarcastic suggestion that he has Ewing's tumor. Ironically, these men are quite physically healthy but have made themselves mentally sick through their paranoid fears.