An interesting area for sure. There's now good evidence to suggest that both pre and probiotics can be beneficial, particularly in reducing URTI incidence (mechanisms: enhanced phagocytic capacity + activity, higher production of IgA and IgM immunoglobulins, and cytokines, as well as enhanced gut barrier function). This Cochrane systematic review is a nice read: https://www.ncbi.nlm.nih.gov/pubmed/21901706
. Probiotics (vs. placebo) reduced URTI symptoms by 47% and the average duration of an acute URTI episode by ~2 days.
Less is known about the relevance of all this to athletes and athletic performance, but I am confident the results from general population studies translate and supplementation may in particular help to reduce GI discomfort + infections, as well as the risk of endotoxaemia during exercise in the heat.
Current recommended dose is ~10(10) friendly bacteria. Good food sources include milk, yoghurt, cheese (but concentrations are low). Prebiotics = raw chicory root, garlic, leek.
Of course it goes without saying testing any products with the athlete to establish tolerance is crucial and best done in training as oppose to comp (not that don't already know this - just putting it out there in case anybody else is reading).
Anyway, that's my 10 cents
hope it's useful!