Contagious diffusion is a type of cultural diffusion that involves the spread of ideas, information, and knowledge from person to person through direct contact. It is similar to the spread of a disease or a forest fire, where the idea spreads rapidly from one source to another. The word "contagious" comes from the Latin word "contagio," which means touch, and thus, direct contact is needed for contagious diffusion. Examples of contagious diffusion include the spread of tea and coffee culture, viral internet memes, and the oral spread of religions.

Contagious diffusion is one of six types of cultural diffusion often taught in geography courses, including AP Human Geography. The other types of cultural diffusion are:

• Relocation diffusion: This occurs when an idea is physically carried to a new location by people who migrate.
• Hierarchical diffusion: This occurs when an idea spreads from a person or group of authority to other people or groups.
• Stimulus diffusion: This occurs when an idea is adopted and modified by a culture to fit their specific needs.
• Reverse hierarchical diffusion: This occurs when an idea spreads from lower-class groups to higher-class groups.
• Contagious expansion diffusion: This is a combination of contagious and hierarchical diffusion, where an idea spreads from person to person but is also adopted by people in positions of authority.

In summary, contagious diffusion is a type of cultural diffusion that involves the rapid spread of ideas from person to person through direct contact.