Here's something interesting: "September 11 study shows flu spreads by airplane" (Reuters).
The 2001-02 flu season in the United States was delayed. The best explanation researchers have is the shutdown of air traffic in the United States for several days because of the attacks on September 11, 2001. By limiting people's travel and interactions, the flu virus didn't spread as much as usual. The air travel freeze acted as a natural experiment that confirmed what scientists had long suspected but been unable to prove.
One lesson is that in a pandemic, such as bird flu, it may be necessary to severely restrict air travel until vaccinations and treatments can be developed.
Separately, the shutdown also served as a natural experiment for scientists studying the influence of high-flying aircraft on Earth's climate: "September's Science: Shutdown of airlines aided contrail studies" (Science News).
This reminds me of the kinds of trends they're able to determine in Freakonomics.