Fanaticism occurs when someone is unwilling or unable to accept a differing point of view. You can use the word to describe the endless war between fans of the Boston Red Sox and the New York Yankees.
Fanaticism come from the Latin word fanaticus, a word with a meaning that includes the word mad — in both senses. If you're mad, you might be angry — or you might be insane. Fanaticism involves both kinds of madness. Anger at those who disagree with you, taken so far that there's a desire to stop or even harm the dissenters? That's madness — and also fanaticism.