'Humans should not try to avoid stress any more than they would shun food, love or exercise.' said Dr. Hans Selye, the first physician to document the effects of stress on the body. While here's no question that continuous stress is harmful, several studies suggest that challenging situations in which you're able to rise to the occasion can be good for you. In a 2001 study of 158 hospital nurses, those who faced considerable work demands but coped with the challenge were more likely to say they were in good health than those who felt they couldn't get the job done. Stress that you can manage may also boost immune(免疫的)function. In a study at the Academic Center for Dentistry in Amsterdam, researchers put volunteers through two stressful experiences. In the first, a timed task that required memorizing a list followed by a short test, subjects believed they had control over the outcome. In the second, they weren't in control: They had to sit through a gory(血淋淋的)video on surgical procedures. Those who did go on the memory test had an increase in levels of immunoglobulin(免疫球蛋白)A, an antibody that's the body's first line of defense against germs. The video-watchers experienced a downturn in the antibody. Stress prompts the body to produce certain stress hormones. In short bursts these hormones have a positive effect, including improved memory function. 'They can help nerve cells handle information and put it into storage,' says Dr. Bruce McEwen of Rockefeller University in New York. But in the long run these hormones can have a harmful effect on the body and brain. 'Sustained stress is not good for you,' says Richard Morimoto, a researcher at Northwestern University in Illinois studying the effects of stress on longevity (长寿),'It's the occasional burst of stress or brief exposure to stress that could be protective. '(325w) The passage is mainly about ______.