Psychological illnesses can be passed from one person to
another just like contagious physical illnesses…

Having too much to do with skeptics may make one start to
doubt. And listening to the words of the fearful may bring
on fear. The same goes for worry, anxiety and suspicions,
jealousy and lust, which can all be passed on through being in close contact and association with others and exchanging facts and communicating with them.

Therefore it is necessary for a person to choose his friends.
And it is not only psychological illnesses which can be
spread by contagion, but spiritual illnesses too!

—H.H. Pope Shenouda III, Experiences in Life