plague
In a place where people over populated, there will be easy spread of air born diseases like Ebola, Tuberculosis among others.
Globalization increased international contact allows diseases to spread farther and faster.
epidemic
New York due to the amount of people living their. Rats infested the place forcing them to recycle to stop the spread diseases
if the disease can't be spread than how did the infected person get it in the first place? unless you conside alcoholism, birth defects (such as congenital muscular torticollis), ect. diseases, its impossible.
By people traveling place to place
It spread by the fleas on rats biting people
Well mainly because someone with a poor hygiene is more likely to have a unhealthy immune system. And as you know someone with a poor immune system is an easy target for diseases to infect. And since we are talking about a whole community of people; so if one of them catch it, it will spread faster in that country than it will in a place with a good hygiene.
Missionarries!
If you're looking to have your kids vaccinated for childhood diseases the best place to take them is to your own doctor. Your doctor can vaccinate them for diseases such as chicken pox, measles, and polio among others.
people thought it was a place for the plague to spread because it was a social place
so that there would be more people to join the muzlims on there journey to the holy place