Final answer:
Contagious diffusion is a rapid form of expansion diffusion, akin to the spread of diseases, where something permeates through a population via person-to-person contact. It contrasts with point source spread by involving longer outbreaks and no single source for infections.
Step-by-step explanation:
Contagious diffusion is a type of expansion diffusion where ideas, phenomena, or infections spread rapidly through a population by contact from person to person. Analogous to the way diseases spread, it involves no movement on the part of the carriers but instead, the idea or disease moves from one host to another, thereby propagating itself similar to a chain reaction. An example of contagious diffusion is a religion spreading across a community as individuals who practice the religion interact with others and convert them.
Moreover, contagious diffusion contrasts with point source spread, which is a form of common source spread wherein transmission is brief and localized. On the other hand, propagated spread, similar to contagious diffusion, involves direct or indirect transmission that can result in more prolonged outbreaks. Just like the molecules of ammonia diffusing in a room, ideas or diseases can spread across a population until they are evenly distributed.