Which groups of people were most closely associated with the spread of the black death?

I know for sure one group would be merchants. They were the ones travelling to potentially infected areas---buying things with the plague's germs, and selling it to people on areas that WEREN'T previously infected. Sailors too I believe, for very similar reasons and because rats were infected, and rats caught rides on ships and spread the disease wherever the ship docked.

Hope this helped. :)