Complications such as multiorgan failure, cardiac dilatation, arrhythmias, ventilator-associated pneumonia, pulmonary hemorrhage, pneumothorax, sepsis syndrome, Reye's syndrome and pancytopenia have been described. A very high mortality of more than 60% has been reported though the risk factors for severe disease are not clear. In 1997, old age, delayed hospitalization and lower respiratory infection were found to be associated with severe disease with children less than 6 years having milder disease. However, recent H5N1 infections have caused high mortality rates in infants and young children. Knowledge related to epidemiology and clinical features remains incomplete and coordination between affected countries is needed to fully understand the profi le of this new viral infection.