If you are sad, and pass a child who is laughing, the child, seeing your sad face, will cease to laugh, not knowing why.

‘Abdu’l-Bahá, ‘Abdu’l-Bahá in London, p. 124

One must protect and safeguard the blessed souls from the breaths and fatal spiritual diseases; otherwise violation, like the plague, will become a contagion and all will perish.

‘Abdu’l-Bahá, Bahá’í World Faith, p. 437

If you put a leper in a room with healthy people, he cannot catch their health; on the contrary they are very likely to catch his horrible ailment.

Shoghi Effendi, Lights of Guidance, p. 188