Question

How was the Black Death spread?

Answer 1

Normally by movement of peoples and trading exchanges.

Explanation:

Black Death or Plague is a disease that is transmitted from rats to humans through the bite of fleas. The flea bites the rat and then bites the human transmitting the infected blood. Once one person is infected it is enough to spit (or sneeze) or touch other persons (sweat and bodily fluids exchange) to transmit the disease.
During the ancient times and the middle age the disease was transmitted through movement of people and goods:
The main carriers were:

1) Armies: waging war was and still is one of the favorite activities of humans. Soldiers got infected in place A and travelling to conquer place B transported the disease. Army quarters and camps (during sieges, for example) were ideal places for proliferation of rats/fleas due to the high concentration of people and poor sanitation (excrements/dead bodies/rubbish).

2) Commerce: people and goods could harbor either the disease (inside the human body of a merchant travelling from one place to another) or the fleas (inside pieces of precious fabric, for example) and, as you know, ships, used for trading, were notoriously infested by rats! It is not a coincidence that the main locations of the starting of the disease in a specific region were ports.