No human is more indestructible than an 18-year old especially when it comes to infectious disease. An 18-year old has a matured and educated immune system backed up by a body in the peak of health. Therefore when we encounter a disease that strikes teenagers, it raises a serious warning flag.
This is why this report [h/t Instapundit] that the current Swine flu attacks teenagers should cause concern. (If true. There is so much noise in the epidemiology and reporting on Swine flu that everything needs to be taken with a grain of salt.)
The Great Influenza of 1918 also showed this pattern. Normal influenza kills infants and the elderly while being no more than an annoyance for everyone else. The 1918 influenza first tore through the military camps of WWI and people initially thought it was just one more of the many diseases that had traditionally ravaged armies. (Prior to WWII, roughly 60% of American wartime in-service deaths resulted from communicable diseases.) When it struck the general population, however, it soon became clear that it targeted teenagers and young adults.
One hypothesis explains this pattern by postulating that the influenza provoked an auto-immune reaction that turned the strong immune systems of young adults against their own bodies. In that case, the stronger a person’s immune system, the more damage the disease did. The current Swine flu might provoke a similar immune response.
Unfortunately, there’s not much we can do about the Swine flu except work faster to create a vaccine and inoculate as many people as possible before the true flu season strikes late next fall. Flu usually kills around 30,000 people in the U.S. every year, mostly the very old. It would be a horrific tragedy to lose 30,000 young people to disease in this day and age.