Plagues

Plagues have affected the world for as far back as we can remember.A running theme though with diseases is that they have always correlated with how humans have maintained their relationship with their habitat. For example the black death. Humans in europe did not keep their environment clean. They had their trash and feces in one large unmaintained place. This would cause rats to congregate in populated places. These rats had fleas which carried the which killed a large population in europe. When more people were dying due to the plague, people had to put the bodies somewhere. They put the bodies in mass graves close to more populated areas infecting more people.
Before the black plague struck, people in the middle ages in Europe did not dispose of their garbage correctly. It was too close to the main population and people could get sick from it. Soon rats and other rodents would go there to feed. They would go back and forth from the populated place to the dump several times carrying fleas which gave the rats the disease. Soon the rats would venture farther into the populated areas coming in contact with humans. Then the humans would spread the disease and from there the plague spread.