Studies have shown that doctor's coats worn in hospitals can harbor contagions including MRSA. Some commentators have likened the transmission of memes to the spread of contagions. Persons most susceptible to suicide contagions are those under 25 years of age. Examples of complex contagions can be copying risky behaviour or joining social movements and riots. Centola is the author of "How Behavior Spreads&colon; The Science of Complex Contagions". However, for complex contagions, weak ties and small worlds can slow diffusion. Centola is best known for his work on complex contagions, collective intelligence, and experimental sociology.