Have you ever noticed how when one person yawns, the people around him tend to do so as well? This phenomenon can be described as a contagion, the spreading of an emotional or mental state (in this case, fatigue).
Contagion can apply not only to the spread of emotions but also to the spread of disease. If you’re feeling sick, you should stay home to reduce the risk of contagion. (Be sure to use the word contagion when you call in sick; it’s a great opportunity to impress people with your vocabulary.) Contagion is akin to the word contagious, an adjective describing things that spread from person to person, like certain diseases... and yawning.