It has been proven by animal research and human clinical trials that the cytokines of especially the innate immune response, IL-1, IL-6 and TNFα, but also the TH1 cytokine IFN-γ induce “sickness behavior” characterized by the well-known symptoms of illness, such as lethargy, depression, loss of appetite and reduced grooming. The gene discussed is IL1A; the disease is major depressive disorder.