An overreaction of the immune cells and cytokines (chemicals that elicit white cell activities) that can severely damage tissues including the lungs, resulting in mild to severe consequences. It is thought that this is one of the primary causes of death of the 1918-1919 pandemic influenza that killed over 40 million individuals globally. The body produced a cytokine storm in the lungs destroying lung tissue and causing massive hemorrhage while attempting to stop progression of the viral infection.