Many emerging diseases (AIDS, SARS, avian flu) are related to viral zoonoses, which have crossed the interspecies barrier. The improvement of epidemiological surveillance has allowed live monitoring of epidemic progresses, while new molecular biological tools have allowed analysis of the various steps required for viral adaptation to their new hosts. These observations suggest lower transmissibility of viral strains at the start of epidemics. It is therefore essential to strengthen public health surveillance systems, so that swift moves can be made before viruses get adapted to humans. Early control measures will be more efficient.