The researchers were attempting to find new methods to treat Cryptosporidiosis, a major cause of infant diarrhea, especially in the developing world, that's brought on by a parasite called Cryptosporidium parvum. They gave the mice Lactobacillus reuteri, a commercially available probiotic supplement, to see if the probiotics were able to shift the bacterial ecosystem back to its more normal, healthy state. A properly functioning microbiome should be able to kill off the invader, since it deprives the parasite of a niche in which to live. Without a spot to take hold in, it eventually dies off. But instead, the parasite actually gained momentum. By the end of the study the researchers found more parasites in the mice that received the probiotic than in those that didn't.