Fruits like apples are known to create a stronger immune system, which in turn can decrease the risk of diseases like the flu. Apples are rich in soluble fiber like pectin which may reduce the inflammation associated with obesity-related diseases and strengthen the immune system. According to a study from the University of Illinois, lab animals fed a low-fat diet with either soluble or insoluble fiber showed distinctly different responses when their immune systems were challenged, with the soluble-fiber-fed animals displaying the less sickness and a faster recovery rate than the other animals. In fact, it was later observed that the results were same when the same tests were conducted on humans.