Deliberately infecting people with nasty diseases in the name of science isn’t a new idea: human challenge trials have helped us understand and treat diseases such as typhoid for decades. But they have long existed on the margins because they were too rarely used for most researchers to be aware of them, as well as due to ethical concerns. Now, that’s beginning to shift. The urgency of the covid-19 pandemic adjusted the calculus of many scientists and policy-makers, bringing fresh attention to the idea of challenging people with some of the world’s most persistent pathogens and diseases, like norovirus and malaria.