Let's also keep in mind that different viral infections manifest varying symptoms, so each strain has it's own peculiar epidemiology. H5N1 seems to be more fatal/devastating to people with the healthiest and strongest immune systems, because this virus overstimulates the body's immune system response, and the various components that are triggered (such as histamine response) overwhelm the cardio-pulminary and respiratory systems. Those whose immune response is diminished, as with older but not yet geriatric victims, seem to have the best rate of survival. Males in their twenties seem to be at greatest risk.