I am curious about the origin of the swine flu currently radiating out from Mexico. The CDC thinks it arose from one individual who was superinfected with both human and swine flu varieties, which then exchanged genetic information.
It certainly would not surprise me if this was simply the latest monster disease to emerge from the factory farming of meat. Packing together unhealthy, antibiotic-marinated animals in proximity with human workers is pretty much the most efficient possible incubator for novel pathogens. While it must be acknowledged that even the most responsible forms of agriculture raise risks of disease evolution and transmission, the characteristics of contemporary factory farming make it much more likely. A notable previous example is MRSA: a disease that seems to have emerged from pig farms in the Netherlands, and which now kills more people per year in North America than AIDS does.
[Update: 4 May 2009] Two updates: Firstly, the text on the Wikipedia page for swine flu no longer includes the text about the CDC I mentioned in my original post. The older Wikipedia text is available here.
Secondly, there is now an article in Newsweek that affirms a link between factory farming and the swine flu epidemic. According to the article: “This virus has been evolving for a long time, no doubt aided in its transformation by the ecology of industrial-scale pig farming in North America.”
[Update: 5 November 2009] Six months after the outbreak started, it appears that not much effort is being put into discovering exactly where the virus came from, or how it passed into the human population.