Some people go out of their way to do something nice for another person, even when it comes at a cost to themselves. Microbes are known to sometimes affect the behavior of their hosts. For example, the rabies virus increases aggressive behavior in infected individuals. Research also shows that the microbiome inhabiting our gut manipulates our social behavior by infecting neurons and altering neurotransmitter and hormone activity. Now researchers at Tel-Aviv University in Israel derive a theory showing that microbes could influence us to act altruistically. The researchers simulations show that microbes may promote the evolution of altruistic behavior in a population to an even greater extent than genetic factors do.