Will getting swine flu now protect me later? Why is winter flu season? Why do vaccines take so long?

The World Health Organization upgraded its swine flu alert status from 4 to 5 on Wednesday, declaring the outbreak a full-blown pandemic. As of Friday, there were 141 confirmed cases in the United States and 331 cases worldwide. With the rising numbers come new questions:

If I get swine flu this year, am I more likely to survive a more serious outbreak next year?

Yes. Getting sick with the flu—or even just being exposed to it—is a lot like getting vaccinated. It doesn't guarantee future immunity, but it does lessen the chances of getting sick again. Vaccines, however, are a more reliable way to fend off sickness, since they are designed to protect you against future strains. (Scientists can predict how the virus will evolve and design vaccines.) Getting sick now, meanwhile, merely protects you from the virus's current incarnation.

There are no hard numbers for how often exposure grants immunity, but if vaccination success rates are any indication, the odds will vary by season and by strain. Flu vaccines typically succeed—i.e., they prevent you from getting the flu—between 70 percent and 90 percent of the time. In 2007, however, the rate went down to 44 percent.

What about seasonal flu? If I'm exposed to that, does that help ward off swine flu?

No. While past studies have shown that seasonal flu vaccines may help ward off avian flu, the Centers for Disease Control and Prevention announced last week that they don't work against swine flu.

Why is flu season in winter?

Because the virus is more stable and stays airborne longer when it's cold. Cells infected with flu virus are coated in a fatty material that hardens and protects them in low temperatures. (When you inhale a flu particle, that coating melts in the respiratory tract, releasing the virus.) Viruses also stay in the air more easily when the air is cold and dry. When it's humid, by contrast, flu-carrying vapor droplets get heavier and fall to the ground.

Temperature makes a huge difference in transmission rates. In a study published in 2007, flu researchers exposed guinea pigs to a virus at various temperatures. They found that transmission levels were high at 41 degrees Fahrenheit but declined at increased temperatures. At 86 degrees, the virus did not transmit at all. (Slate's Andy Bowers tackled this question for NPR in 2003.)

The last major swine flu outbreak was in 1976. Where has swine flu been since then?