An elevated body temperature can kill. Yet fever?which is precisely that?is a common response to infection. Recent research shows how febrile temperatures encourage a particular protein called Hsp90 to shepherd immune-system cells to sites of infection. This research suggests ways in which the process might be regulated to a patient?s advantage. Drugs that increase Hsp90 production should promote the migration of T-cells to lymph nodes, and so aid the treatment of infections that need a larger immune response. Conversely, drugs that diminish Hsp90 production might help reduce T-cell movement in people with so-called auto-immune disorders, such as multiple sclerosis and rheumatoid arthritis, in which immune-system cells attack the body they are part of.