Many things are said to be infectious. Sleepiness can be infectious, and yawning as well. In large-scale strategy, when the enemy is agitated and shows an inclination to rush, do not mind in the least. Make a show of complete calmness, and the enemy will be taken by this and will become relaxed. You infect their spirit. You can infect them with a carefree, drunklike spirit, with boredom, or even weakness.
A book of five rings, Miyamoto Musashi, seventeenth century.