Yawning is known to be contagious among humans and even some animals. When you see someone yawn, it can trigger a yawn in you, even if you're not tired.
Positive and negative emotions are contagious. Someone who is cheerful can lift your mood, while someone who is anxious or furious might do the same.
Laughter is infectious. Hearing laughter can trigger laughter in others, even if they don't know what's funny.
Stress can be contagious in social situations. Being around someone who is stressed can increase your own stress levels.
People often adopt the attitudes and opinions of those they spend time with, especially in close-knit social groups.
Fashion trends, slang words, and popular behaviors can spread rapidly through social networks, becoming contagious cultural phenomena.
Obesity may be socially contagious, say studies. Having overweight friends or family can raise the risk of obesity.