Immunity and colds/flu. There’s a close connection between the bacteria in your colon and the immune system—and probiotics have been linked to enhanced immune responses (such as to flu vaccines). Several studies, including one published in 2012 in the British Journal of Nutrition, have found that certain probiotic strains boost measures of immune response—but whether this translates into any clinical benefits is uncertain. Studies have been inconsistent, for example, as to whether taking probiotics will actually curb colds and other upper respiratory infections. A 2011 Cochrane review concluded that probiotics may help prevent such infections, though there were limitations in the studies and no data for older people.