Boys predominantly pass on flu to other boys and girls to girls, according to a new study of how swine flu spread in a primary school during the 2009 pandemic. The results also suggest that flu transmission is most intensive between children of the same class, but that sitting next to an infected person does not significantly increase a child’s risk of catching flu. The data will help researchers to model how epidemics spread and how interventions such as school closures can help contain an outbreak.