More research has been done in recent years on the importance of a healthy gut microbiome to support overall health and a strong immune system. My dogs get a probiotic supplement for most of their lives, and it starts at birth. These pups got a dose of probiotic gel (Bene Bac Plus Gel) at birth, and days 3, 5, and 7. They will get it again when they start weaning off mom to solid food (to address weaning stress), and all the pups will go to their new homes with a probiotic tube to support the stress of that transition. So far we have found this protocol works to keep them from getting diarrhea during those times of extra stress.
There are several good probiotics out there, and some individuals do better on different strains. Most recently I’ve used Geneflora for Pets, and another one is a human variety but it works well for my dogs- Garden of Life Primal Defense.