For most of human history, infectious diseases probably killed more people than anything else, said Joshua Ackerman, a social psychologist at the University of Michigan in Ann Arbor. The need to defeat viruses, bacteria and other parasites has shaped well-known elements of the immune system such as antibodies and white blood cells.¶¶But the traditional immune system can only respond once a parasite is inside our bodies. By that time, the invader may already have caused damage, and to destroy it, the body must fight a messy and expensive war.¶¶When possible, it’s better to avoid catching a disease in the first place. So evolution has crafted a parallel immune system in our minds, and at its core is disgust. That “ew” feeling is part of what motivated our ancestors to avoid likely sources of infection such as feces, vomit and rotting food.