"It remains unclear why the virus has killed so many young, healthy adults — a concern because this is characteristic of pandemic strains. One possibility is that the virus triggers a massive immune reaction called a cytokine storm, which is worse in fit people with a strong immune system.

This is thought to have been the chief cause of death during the Spanish Flu of 1918-19, which was caused by an H1N1 strain."

An extract from an informative science article in The Times April 28th
Full article Deadly strain of swine flu gets under radar of the immune system