Economic Stress and swine flu..

Some researchers speculate that in 1918 immune systems were weakened by malnourishment and the stresses of life. The lower socioeconomic groups of Mexico had been very stressed by a collapse in the oil price and hence economic activity generally, the US recession in contruction, which has lead to vastly decreased remittances from family members working in the US.

CDC officials expect to see more severe cases in the U.S. as well - and as better epidemiological work is done in Mexico, we'll probably hear about more mild cases there too.

The true severity of the H1N1 swine flu virus is an open question. The 1918 Spanish flu pandemic began with a fairly mild wave of infections in the spring, but the virus returned a few months later in a far more virulent form. That could happen with the current swine flu as well. "It's quite possible for this virus to evolve," said Fukuda. "When viruses evolve, clearly they can become more dangerous to people."