It doesn't matter what kind of fight you're in-a bar fight, a fight with the mechanic, a family food fight-you always want someone else on your side. Simple math shows that the more people who are on your side, the better your chances. The same goes for your health: creating a strong social network is paramount in raising the level of your immune system. How?
Depression has been linked to infection, presumably because depression inhibits the fighting ability of your immune system's T-cells. It's not just forgetting to activate your burglar alarm; it's not caring whether it's activated or not. Group participation works, too-religious groups, social groups, and work groups are all beneficial.