The development of cities, homes, and farms and other permanent settlements allowed the human population to increase and advance to where it is today. However, that is not without a price. As population density increases sanitation problems rise. Waste management has always been a problem of society. With fecal matter in the streets fecal borne pathogens can be easily spread. Another thing about population density is that a pathogen doesn’t need to travel as far to keep infecting new hosts. Additionally, as we become more sedentary, we also become more mobile as well by trading across long distances. This is how the Black Death was theorized to spread. Traveling from China to Europe along the silk road, the plague was able to travel hundreds of miles where as before people developed cities, such a migration was impossible. Also, as we domesticated and raised more species and in greater quantity our interaction with animals increased allowing zoonotic diseases to more easily spread to humans. This new contact with animals along with increased population density meant that diseases had more chances of developing a mutation that allows it to jump to humans or increases the severity of it. Normally when this would happen when we were hunter gatherers the mutation would die out since it would run out of hosts, but cities and towns provide it a path to expand and travel and thrive.