Participants will provide saliva and stool samples (for the sub-group)during the pre-baseline week and during the week of final exams at approximately week 4. At the randomization visit, height, weight and age will be obtained. Participants will be stratified based on body mass index and randomized via sealed envelopes to one of four groups: Lactobacillus helveticus, Bifidobacterium longum ssp. Infantis, Bifidobacterium bifidum or the placebo (n=145/group). Participants will take the study provided capsule of probiotic or placebo daily for 6 weeks. Participants will receive daily questionnaires via email throughout the prebaseline week and the 6-week intervention. Daily questionnaires will ask about intake of the supplement, cold symptoms, and general health (i.e., new medications, doctor visits). Throughout the study, participants will also be asked to complete weekly questionnaires (inquiring about gastrointestinal symptoms and physical activity) and Quality of Life Questionnaires (administered during pre-baseline, final exam week and week 6 of the study).