The Black Death was an epidemic of bubonic plague, a disease caused by the bacterium Yersinia pestis that circulates among wild rodents where they live in great numbers and density. Such an area is called a âplague focusâ or a âplague reservoirâ.
Diffusion is the net movement of molecules or atoms from a region of high concentration to a region of low concentration.his is also referred to as the movement of a substance down a concentration gradient. A gradient is the change in the value of a quantity (e.g., concentration, pressure, temperature) with the change in another variable (usually distance).
Thermal energy drives chemical diffusion, a process that results from random collisions of atoms, molecules, and particles. This movement drives diffusion. At higher temperatures, the frequency and violence of these motions and collisions increase, speeding up diffusion. Diffusion can occur in each of the three phases: solids, liquids and gases. The mechanisms are related but different in each, and so the effect of temperature is different.