Popular Internet nerd and educator CGP Grey explains how ideas and thoughts spread from person to person in the same way germs spread. By reading or hearing a new idea, you have been ‘infected’ with a new idea.
And if you like the new thought you may very well spread it to your friends.
With the Internet, this phenomenon is greatly amplified. This is all good and fun when we’re talking about cat memes. But when angry and negative ideas spread online things go downhill quickly.