It will surprise no one to learn that interest in swine flu has exploded recently, but now comScore has published some numbers quantifying that explosion. Around 501,000 Americans performed 929,000 swine flu-related web searches during the week ending Sunday, April 26, in both cases an increase of more than 1,500 percent from the previous week, the data firm says.

Naturally, there were advertisers eager to take advantage of that increased interest. The number of firms that paid for ads to display with swine flu-related search results increased from 73 to 271.

Perhaps more interesting than the numbers was the list of companies who took out ads. It was, as comScore puts it, a mix of advertisers interested in “both public and commercial benefit.” The American Lung Association’s Faces of Influenza site, which includes flu information, saw the most ad exposures by far, more than double those for AreYouPrepared.com. The number three site, however, is a bit less flu-related: It’s Microsoft‘s page offering cash for people who use its Live Search service. Also on the list of top advertisers: RightHealth, the health site from search startup Kosmix.

Note that this data ends five days ago, shortly after the swine flu pandemic really started to burst into public consciousness. With new online tools for tracking the disease, an increase in swine flu-related spam, and the swine flu claiming the top three terms in Twitter’s search trends (to pick three random examples), I expect the search activity was even higher this week.