There's nothing more contagious than the laughter of young children; it doesn't even have to matter what they're laughing about.
Yawns are not the only infectious things out there besides germs. Giggles can spread from person to person. So can blushing. But maybe the most powerful infectious thing is the act of speaking the truth.
Aggression unopposed becomes a contagious disease.
I really believed that fear is contagious.
Confidence is contagious. So is lack of confidence.
Freedom is the most contagious virus known to man.
Cruelty is contagious in uncivilized communities.
Like timidity, bravery is also contagious.