All strains of bacteria possess genetic differences; these differences are not sufficient for them to be considered as separate species, but each strain is distinctive. For example, many different strains exist of E. coli. Some, such as E. coli 0157:H7, cause serious diseases, while others live in the intestine and can be considered beneficial because they aid digestion. In fact, although billions of bacteria exist in the world, less than 1 percent of all bacteria cause disease.