Flu has a history of foreshocks

H1N1 swine flu may be fading from the news some, but the number of confirmed cases nationwide has been higher than is usual for seasonal flu in the month of May. What does that mean? Is this flu's ability to linger into the spring suggestive of how different a beast it is? And what does it portend for how the virus might infect Americans over the summer and come fall?

The Centers for Disease Control and Prevention publishes confirmed cases of the novel H1N1 swine flu virus each week. The official count of probable cases in the U.S. as of Friday was 8,975 in 49 states with 15 deaths. However, the total number of people infected in the country is estimated to be as high as 100,000, says CDC spokesman Joe Quimby, taking into account those who were sick but didn't seek medical care.

For the most part, the H1N1 virus doesn't act very differently from our contemporary flu strains in terms of how sick people get and how easily it is transmitted from person to person, says Ira Longini, an expert in modeling the spread of infectious disease. Its lingering presence in the spring is probably because nearly the entire human population is susceptible to infection with it.

"Usually, we have some protection, even [in] children, due to past illness or vaccination," Longini says. But with swine flu, that's not the case. "So it's going to spread very easily when it's introduced into families and schools," causing sickness even at a time outside of the flu's normal season.

Merely an omen?

Why seasonal flu winds down every spring is not entirely clear, but contributing factors may include a change in climate -- flu viruses survive longer in cooler, drier air -- and a dwindling number of susceptible people. The spread of swine flu may be interrupted by warmer weather and by schools letting out for summer.

Or it may not. "When you have a highly transmissible, immunologically novel virus, you can have widespread epidemics throughout summer," says Don Olson, research director at the nonprofit International Society for Disease Surveillance in New York. "Our population level of immunity is just poor enough that we're kind of like dry wood."

In 1957, for example, there were summer outbreaks of the so-called Asian flu that would later become a pandemic and cause nearly 70,000 U.S. deaths. Several outbreaks were well-documented and included a church gathering of international students in Grinnell, Iowa, and schoolchildren in Louisiana, who were attending school in August that year.