“You are the average of the five people you spend the most time with,” is a famous motivational quote by Jim Rohn.
Who you associate with, whether in your personal and/or professional life has an impact on you and your results. Consider when you’re at an event and everyone is happy, truly enjoying the scene. Contrast that with coming upon a protest. You may or may not agree with the theme of the gathering, but you’ll immediately pick up on the energy of the people. 
This is why, in the game of power, you must protect your mindset, your energy and your focus. You do so by surrounding yourself with those who value and strive for the same or similar ideals…and you actively avoid being infected by those who do not.
~ Courtesy of The 48 Laws of Power by Robert Greene
There are many kinds of infector to be aware of, but one of the most insidious is the sufferer from chronic dissatisfaction. Cassius, the Roman conspirator against Julius Caesar, had the discontent that comes from deep envy. He simply could not endure the presence of anyone of greater talent.
Probably because Caesar sensed the man’s interminable sourness, he passed him up for the position of first praetorship, and gave the position to Brutus instead. Cassius brooded and brooded, his hatred for Caesar becoming pathological. Brutus himself, a devoted republican, disliked Caesar’s dictatorship; had he had the patience to wait, he would have become the first man in Rome after Caesars’s death, and could have undone the evil that the leader had wrought.
But Cassius infected him with his own rancor, bending his ear daily with tales of Caesar’s eveil. He finally won Brutus over to the consipiracy. It was the beginning of a great tragedy. How many misfortunes could have been avoided had Brutus learned to fear the power of infection.
There is only one solution to infection: quarantine.