Researchers: Optimism improves immune system

The study, published in the journal Psychological Science, followed the outlook of students over the course of a year and tested their immune system during that period, Telegraph reported.

A team of researchers, led by Professor Suzanne Segerstrom, a psychologist at University of Kentucky, asked 124 first-year law students to fill in five questionnaires measuring optimism by asking how closely they identified with statements such as "I will be less successful than most of my classmates," and go through immunity checks over the course of a year.

The immunity tests consisted of the students being injected with dead mumps virus or candida yeast. These harmless shots triggered a cellular immune response, resulting in a small bump at the injection site. 

Researchers estimated the strength of the immune response by measuring the bump.

As the students experienced different levels of stress during classes and exams, their levels of optimism changed and so did their immunity.

The more optimist they were, the stronger their immune response proved to be. When they felt more pessimistic, their immune system weakened.