Begin reading for Topic $2$ as well as listening to the audio lectures $2$A and $2B$ as per the syllabus $($chronologies$,$ readings and web document on the Mummy's Medicine Chest under course documents$-$please note that the web document and the reading on the Mummy's Medicine Chest are the same$)$ AND answer the following question:

Civilization and writing is believed to have begun with the first cities of the world around $3,000$ BCE in river valleys around the world $($Nile River Valley, Tigris Euphrates, indus, and Yellow River Valley$).$ Along with civilization and writing. we also see the start of epidemic disease as these first cities provided the environment for disease to begin and spread. What were the factors that helped create the first epidemics and which factors were the most important?