The probiotic Lactobacillus substantially cuts the rate and length of respiratory illness in professional long distance runners, reveals a small study. Intensive exercise can subdue the normal immune response, and as a result, some athletes are vulnerable to respiratory viruses, such as colds and flu. During four months of intensive winter training, 20 elite, endurance athletes were given either three freeze dried capsules twice daily of the probiotic Lactobacillus fermentum or a dummy capsule (placebo).