The timing of flu season is a little hard to predict．Part of what makes it unpredictable is that scientists still don t understand exactly why we have one at all．There have been lots of theories: One theory is that people spend more time indoors, with the windows closed, breathing each other s air．Other scientists have argued that cold of winter weakens our immune systems．A third theory is that the flu virus lives in the cold, dry air, but suffers in the warm, humid air．For a while, scientists had a hard time testing these theories: they needed to run experiments, but researchers aren t allowed to infect humans with illnesses, and most lab animals arent affected by the flu the same way people are．In 2007, a medical researcher named Peter Palese found an 80-year-old journal article that reported that guinea pigs (豚鼠)get infected and pass on the flu just like humans．Palese decided to test Theory 3．The research team led by Palese ran several experiments and in each experiment, they injected half the guinea pigs with influenza A (the common flu), and put them in a box next to a box of uninfected animals．At a temperature of 41 degrees, all four of the exposed guinea pigs caught the flu, but when Palese repeated the experiment at 68 degrees, only one of the animals was infected．And when he ran the test at 86 degrees, none of the exposed animals got sick．The researchers also ran experiments where they varied the humidity in the room but kept the temperature constant: the drier the air, they found, the more animals got sick．Palese s study showed that the influenza virus does spread more effectively in cold, dry air.
What does Paragraph 3 mainly discuss?