The recent CDC report on identification of a coronavirus as the cause of SARS does not provide much clarity but rather adds to the confusion regarding the nature of the infectious agent responsible for SARS. The virus identified at HKU is also a coronavirus, but identification at the Chinese University of Hong Kong, in Marburg and in Ontario differ from that particular identification. The candidates so far include, an unspecified paramyxovirus, the recently discovered human metapneumovirus (representing the two subfamilies of the family Paramyxoviridae, and now an unspecified coronavirus (representing the family Coronaviridae). A common feature of the families Paramyxoviridae and Coronaviridae is that they contain viruses that are ubiquitous respiratory tract pathogens. Consequently it would not be unexpected to detect such viruses in clinical samples entirely fortuitously, particularly where a diagnostic technique of limited discriminatory potential such as electron microscopy is employed. The paramyxoviruses and coronaviruses are quite distinct viruses with different biological potential. The paramyxoviruses are negative-sense RNA viruses, which do not recombine, whereas the coronaviruses are positive-sense RNA viruses, which undergo recombination at high frequency. This makes these viruses particularly good candidates as emerging pathogens. In the UK at least coronaviruses are second only to rhinoviruses as causes of the common cold. In the case of the four types of human parainfluenza viruses (sub-family Paramyxovirinae), human respiratory syncytial virus and human metapneumovirus (sub-family Pneumovirinae) infection occurs in infancy and early childhood, but does not confer prolonged immunity. In the case of human respiratory syncytial virus it has been estimated that reinfection occurs on average every 36 months. The outcome of infection can be serious in infancy (bronchiolitis) and in the elderly (pneumonia), At other ages infection is usually asymptomatic or mild upper respiratory tract infection (common cold). The recently discovered human metapneumovirus appears to behave similarly, but is less prevalent than respiratory syncytial virus, as in the reference provided at our site on March 22nd, 2003. These authors conclude that: "Human metapneumovirus infection occurs in adults of all ages and may account for a significant portion of persons hospitalized with respiratory infections during some years."

Continued caution is required in evaluation of the competing claims of identification of the etiologic agent of SARS.

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