A study into swine-flu transmission has found that boys will generally infect boys, while girls mainly pass the virus to other girls.
The study by the British Medical Research Council's Centre for Outbreak Analysis and Modelling at Imperial College London found children were three times more likely to transmit the infection to others of their own gender.
Transmission rates are also five times higher between classmates than those in different classes.
The findings could help schools decide how to control an outbreak in the classroom.