This diagram shows the power that the germ theory provided to physicians and public health leaders. The central concept, that one germ causes one disease, led to interventions that actually stopped epidemics. For example, quarantines for some diseases like smallpox, came to be understood as methods to prevent person-to-person transmission of microbes. The discovery of vectors such as water, milk, insects, and healthy human carriers permitted public health officials to clean up water supplies, kill mosquitoes, and promote the use of sanitary privies to attack diseases via their method of transmission. A rudimentary understanding of immunity fostered the production of preventive vaccines. And as a final step, studies of microbes and their life cycles allowed the development of diagnostic tests and even some effective therapies. Anti-diphtheria serum was the first dramatic "cure" for a disease. Parents who had previously watched their children slowly suffocate from diphtheria before the antiserum and watched the child recover rapidly after being given the antiserum were astounded at the new power of medicine.