SARS is associated with epithelial-cell proliferation and an increase in macrophages in the lung. The presence of haemophagocytosis supports the contention that cytokine dysregulation may account, at least partly, for the severity of the clinical disease. The case definition of SARS should acknowledge the range of lung pathology associated with this disease.

Replication in SARS-CoV-infected macaques of pneumonia similar to that in human beings with SARS, combined with the high prevalence of SARS-CoV infection in SARS patients, fulfill the criteria required to prove that SARS-CoV is the primary cause of SARS.

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NoV epidemics resulted from the global spread of GII.4 strains that evolved under the influence of population immunity. Lineages show notable (and currently unexplained) differences in geographic prevalence. Establishing a global NoV network by which data on strains with the potential to cause pandemics can be rapidly exchanged may lead to improved prevention and intervention strategies.

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In 1997, 18 cases of influenza in Hong Kong (bird flu) caused by a novel H5N1 (chicken) virus resulted in the deaths of six individuals and once again raised the specter of a potentially devastating influenza pandemic. Slaughter of the poultry in the live bird markets removed the source of infection and no further human cases of H5N1 infection have occurred. In March 1999, however, a new pandemic threat appeared when influenza A H9N2 viruses infected two children in Hong Kong. These two virus isolates are similar to an H9N2 virus isolated from a quail in Hong Kong in late 1997. Although differing in their surface hemagglutinin and neuraminidase components, a notable feature of these H9N2 viruses is that the six genes encoding the internal components of the virus are similar to those of the 1997 H5N1 human and avian isolates. This common feature emphasizes the apparent propensity of avian viruses with this genetic complement to infect humans and highlights the potential for the emergence of a novel human pathogen.

Avian H9N2 influenza A virus has caused repeated human infections in Asia since 1998. Here we report that an H9N2 influenza virus infected a 5-year-old child in Hong Kong in 2003. To identify the possible source of the infection, the human isolate and other H9N2 influenza viruses isolated from Hong Kong poultry markets from January to October 2003 were genetically and antigenically characterized. The findings of this study show that the human H9N2 influenza virus, A/Hong Kong/2108/03, is of purely avian origin and is closely related to some viruses circulating in poultry in the markets of Hong Kong. The continued presence of H9N2 influenza viruses in poultry markets in southern China increases the likelihood of avian-to-human interspecies transmission.