Why are military bases considered "ground zero" for epidemic outbreaks?

The military basis are considered as ''ground zero'' for epidemic outbreaks because of the constant movement of the people. The soldiers are moved from one place to another regularly. They can be in a military base in Africa, than sent in a military base in Asia, after that get back home. Wherever they are, they are in constant touch with people. The diseases that are found in certain places in the world can be very easily transferred to them, and they will easily transfer the disease to their colleagues, to the people in another military base, or to the people back home, thus being the ones that spread out the disease and cause an epidemic of the same.

your answer would be (a)