Mental states can have a profound impact on how ill we get and how well we recover. Understanding this could help to boost the placebo effect, destroy cancers, enhance responses to vaccination and even re-evaluate illnesses that, for centuries, have been dismissed as being psychologically driven. Neuroscientist Catherine Dulac and her team at Harvard University in Cambridge, Massachusetts, have pinpointed neurons in an area called the hypothalamus that control symptoms including fever, warmth-seeking and loss of appetite in response to infection. "Most people probably assume that when you feel sick, it's because the bacteria or viruses are messing up your body," she says. But her team demonstrated that activating these neurons could generate symptoms of sickness even in the absence of a pathogen. An open question, Dulac adds, is whether these hypothalamic neurons can be activated by triggers other than pathogens, such as chronic inflammation. The insula ... is involved in processing emotion and bodily sensations. A 2021 study ... found that neurons in the insula store memories of past bouts of gut inflammation " and that stimulating those brain cells reactivated the immune response. Such a reaction might prime the body to fight potential threats. But these reactions could also backfire. This could be the case for certain conditions, such as irritable bowel syndrome, that can be exacerbated by negative psychological states.