In slightly more medical terms, the Spanish flu triggered what’s called a cytokine storm. Cytokine proteins exist in your body to modulate the release of immune cells when there is an infection. A healthy immune system has a lot of those little fellows. In a cytokine storm, too many immune cells flood the site of the infection, which causes inflammation around that site. If the site of the infection is in the lungs—as it could be in a respiratory disease like the Spanish flu—the inflamed lungs fill with fluids. Then you die.
A very good description of the pathogenesis of flu
