The infection may not be obvious if it does not produce other visible results so it may have been considered benign by itself, the disease affects the brain but

The infection may not be obvious if it does not produce other visible results so it may have been considered benign by itself, the disease affects the brain but the infection is in the gut, the disease looks very much like others that are not caused by bacterial infections (instead: genetic disorders, poisoning or prions) ... so while it is indeed simple, it's rather tricky and misleading.