Q: Why is it more deadly in Mexico than anywhere else it’s surfaced?
CNN: The short answer is investigators don’t know. The deaths have occurred in healthy people, as opposed to those usually most at risk from the flu: the young, the old and those with compromised immune systems. The same thing happened in recent years with the SARS and avian flu outbreaks. The spreading virus starts a cascade within the body as the immune system overreacts. Fluid builds up in the lungs and inflammatory cells throughout the body react to the infection.
The rest of the answer is about other viruses better understood in hindsight, and might apply if current patterns in Mexico are better understood. Which they aren't yet.
So, there may be a lot more people in Mexico that have been infected but they have not been detected because they did not end in a hospital.