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).
Diffusion refers to the net movement of molecules from higher to lower concentration. Diffusion may be a simple diffusion or . A simple diffusion is one in which that occurs unassisted. Facilitated diffusion in contrast is an assisted diffusion in a way that it requires a carrier molecule. For instance, polar molecules diffuse across a cell membrane through carrier molecules in the cell membrane.