Emotions are contagious. They are passed on automatically, rapidly and frequently from one person to another. Such contagion is at the heart of empathy, social connections and relationships between people. Anger felt by a friend for an injustice or joy for an exam easily spill on to us. We often mime the expressions, posture and voice of the people with whom we interact. Emotional contagion allows us to transfer important info and the exchange of news is what permitted the human race to survive. The spread of social networks reveals that emotional contagion does not require that those who are ‘infected’ see or speak with one another or have direct contacts. Contagion can be good or bad and those involved may equally well be involved in charity work or in violence. Some emotions such as joy and compassion are more contagious whereas others such arrogance less. Our moods can be sent through networks without temporal or geographic barriers posing any limitations. They can influence fashion, ideas, and behaviors of entire social groups. The fact that my neighbor is obese, smokes, goes jogging divorces etc. only minimally affects the chances that I copy these behaviors. However, if it is my friend who does these things the likelihood increases considerably. The chances fall if it is the friend of my friend who does so and even more if it is the friend of the friend of the friend. The chances are comparable to those of my neighbor after four levels of friendship. To belong to a network is written in our genes and the members have special power just like carbon atoms that depending on how they are connected form either carbon or diamonds. Studies attempt to throw light on the laws that rule these networks. What exactly goes down a network of thousands of people who decide to get tatoos? Do the properties of a network depend on the connections or on the points of connection? Answering these questions means having a control over the network and its members.