Control of many infectious diseases relies on the detection of clinical cases and the isolation, removal or treatment of cases and their contacts. The success of such ‘reactive’ strategies is influenced by the fraction of transmission occurring before symptoms appear. We performed experimental studies of foot-and-mouth disease transmission in cattle and estimated this fraction at less than half the value expected from detecting virus in body fluids, the standard proxy measure of infectiousness. This is because the infectious period is shorter (mean 1.7 days) than currently realised and animals are not infectious until, on average, 0.5 days after clinical signs appear. These results imply that controversial pre-emptive control measures may be unnecessary; instead, efforts should be directed at early detection of infection and rapid intervention.