5 weekly aerobic sessions of about 20 minutes each seemed to produce the best results. Those who _________________________. Nieman says the explanation comes from what happens to certain white blood cells when the body kicks into exercise mode: _________________________, looking for invaders, such as the viruses that cause colds and other respiratory infections.

"Most immunologists interpret this as what they call improved immunosurveillance, or the ability of the immune system to detect and then engage pathogens and destroy them."

The so-called common cold is one of the most frequent illnesses in many parts of the world. According to one study, the economic burden is_________________________. David Nieman's study is reported in the British Journal of Sports Medicine.

(1)exercised less frequently were more likely to catch cold
(2)these immune cells start roaming the blood stream
(3)estimated at $40,000,000,000 a year in the U.S. alone