How did the rise of agriculture help bring about epidemics?

The rise of agriculture began 11,000 years ago in what is often called the Fertile Crescent, a region of the Middle East that then possessed productive soil and abundant water sources. Some of humankind’s most significant infectious diseases also emerged around 11,000 years ago, leading many researchers to infer a correlation between epidemics and the transition from hunter-gatherer societies to agricultural ones.

Several physiological and social factors that promote the spread of epidemics have been traced back to humankind’s initial permanent settlements. Two of the most commonly cited contributing factors to the advent of epidemics are less nutritional variety and greater population density. In order to cultivate crops, farmers saved and replanted seeds. To increase crop yields, they learned to save only the seeds from their most productive, favorable plants. This artificial selection decreased the variability of the crop and in turn the variability of the diet. Some researchers believe the loss of diversity led to nutritional deficiencies and dental disease, which put new stresses on the body and made it more susceptible to infection.

Crowd epidemic diseases spread faster in dense populations. Hunter-gatherer societies each consisted of only a few dozen people. If a member of the group acquired an infectious disease, they could only infect a small number of people—hardly an epidemic. Once the rise of agriculture led to larger settlements, the infection potential became larger as well. Animal domestication in agricultural societies also had a role in the emergence of epidemics. More frequent contact with animals likely led to a crossover of animal-to-human pathogenic diseases like smallpox, tuberculosis, diphtheria, measles, mumps, and pertussis, all of which are highly contagious.
https://www.nas.edu/evolution/EvolutioninAgriculture.html

https://www.ncbi.nlm.nih.gov/books/NBK114494/

https://www.ncbi.nlm.nih.gov/pubmed/21507735