More and more research groups are seeking to show that changes in the microbiota of a particular site are involved in diseases, such as periodontal disease, inflammatory bowel disease, and premature...

More and more research groups are seeking to show that changes in the microbiota of a particular site are involved in diseases, such as periodontal disease, inflammatory bowel disease, and premature birth. Critics object that showing an association is not the same as demonstrating cause and effect. In the case of the obesity study, scientists tried to do this by inoculating germ-free mice with different variations of the microbiota. Clearly, in humans, this would not be possible. How might you prove cause and effect in humans?