Disease and the various ways that we have dealt with disease has probably had the biggest impact on humanity. Until humanity started figuring methods of preventing and reducing the impact of disease, the world's population (for better or worse) would be nowhere near the present levels.  We have developed a myriad of responses to preventing and treating disease ranging from things as simple as bathing on a regular basis to very complex technologies and surgical procedures.  Without the medical procedures and technologies that have been developed over the ages, the human population would, most likely, be much, much less than it is today.  Cities as we know them couldn't exist without some methods of treating and preventing disease.