The circulation of numerous influenza virus subtypes in water birds constitutes a continuous threat with respect to emergence of a future influenza pandemic. The interspecies barrier is high but has already been breached 3 times this century, notably in 1918, 1957 and 1968. Still, it is impossible to predict when another pandemic will occur. Continuous and intensive surveillance of influenza viruses in man, pigs, and birds, especially in China, may provide the opportunity to prepare a vaccine timely in sufficient amounts, at least for the industrialised countries.