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).
Neither of these require energy. Diffusion occurs naturally when a higher concentration of a substance exists on one side of a membrane than the other; molecules will enter in â¦ to the area of lesser concentration until there is an equilibrium on both sides. Active transport requires the energy of the cell.