CHRIS PETERSON WAS TEACHING a class in abnormal psychology at Virginia Tech when he told his students to fill out an Attributional Style Questionnaire — a carefully designed test that determines a person’s level of optimism and pessimism. The students also answered questions about their general health, including how often they went to a doctor.

Peterson followed the health of his students the following year and discovered that the pessimists had twice as many infectious diseases and made twice as many trips to the doctor as the optimists.

Later, Martin Seligman of the University of Pennsylvania and two of his colleagues, using interviews and blood tests, found that optimists have better immune activity than pessimists. Studies by other researchers show the same thing. Why? One big factor is that “Pessimistic individuals,” as Seligman writes, “get depressed more easily and more often.”

When a person is depressed, certain brain hormones become depleted, creating a chain of biochemical events that end up slowing down the activity of the immune system. For example, two key players in our immune systems are T cells and NK cells.

T CELLS recognize invaders (like viruses) and make more copies of themselves to kill off the invaders. Pessimists’ T cells don’t multiply as quickly as optimists’, allowing invaders to get the upper hand.

NK CELLS circulate in the blood and kill whatever they come across that they identify as foreign (like cancer cells). Pessimists’ NK cells can identify foreign entities, but they don’t destroy them as well as the optimists’ NK cells.

Optimists also look at information in more depth to find out what they can do about the risk factors. In a study by Lisa Aspinwall, PhD, at the University of Maryland, subjects read health-related information on cancer and other topics. She discovered that optimists spent more time than pessimists reading the severe risk material and they remembered more of it.

“These are people,” says Aspinwall, “who aren’t sitting around wishing things were different. They believe in a better outcome, and that whatever measures they take will help them to heal.” In other words, instead of having their heads in the clouds, optimistic people look. They do more than look, they seek. They aren’t afraid to look into the situation because they’re optimistic. Thus, for yet another reason, optimists are likely to be healthier.

The best news is what research has shown repeatedly: Anyone can become more optimistic with effort. And every effort you make to keep an optimistic attitude will reward you with a stronger immune system. So you’ll enjoy better health. And it is also true that the better your health, the easier it is to maintain an optimistic outlook.