http://www.sciencealert.com/freaky-new- ... -behaviour
"The relationship between people and pathogens, the researchers suggest, could have directly affected the development of our social behaviour, allowing us to engage in the social interactions necessary for the survival of the species while developing ways for our immune systems to protect us from the diseases that accompany those interactions."
From an evolutionary point of view, this makes sense, because social behaviour would be in the interest of pathogens to help allow them to spread. And for us, the social behaviour leads to reproduction and the propagation of the species, so it's a win/win.
To investigate whether this could be the case, in the latest study, the researchers switched off the immune molecule interferon gamma in mice, flies, zebrafish, and rats. Because this molecule runs and tells the rest of the immune system when germs are about, they were testing what would happen when that interaction was shut down.
In all species, they showed that interferon gamma was essential to normal social interaction.
They found that blocking the molecule in mice caused the animals' brains to become overly connected, making the mice less willing to interact with others.