Prolonged Stress Leads to Suppression of the Immune System

Prolonged stress leads to suppression of the immune system. What might explain the evolution of such a seemingly flawed reaction?

A) There is no reasonable explanation for this.
B) Susceptibility to disease would increase the likelihood of an early death.
C) Immune suppression has no lasting effects on overall health.
D) Such immune suppression might prevent the development of autoimmune illnesses.