Immunotherapy may hold promise in fighting COVID-19 or other viruses, according to a study from the National Institutes for Health. NIH investigators have found that when the immune system first responds to infectious agents such as viruses or bacteria, a natural brake on the response prevents overactivation. Their new study in mBio describes this brake and the way pathogens such as SARS-CoV-2, the virus that causes COVID-19, turn it on. Their finding provides a potential target for an immunotherapy that might be applied to a wide range of infectious diseases.