The functioning of the immune system is increasingly seen as the key factor in the development of disease—from cancer to heart disease and from Parkinson’s to Alzheimer’s. Now scientists have found that the immune system also directly affects—and even controls—our social behavior, such as our desire to interact with others. That finding, which overturns decades of textbook teaching that the brain lacks a direct connection to the immune system, has significant implications for neurological diseases such as autism-spectrum disorders and schizophrenia.