During the winter months (Late November to early April), respiratory illnesses caused by both respiratory syncytial virus (RSV) and influenza virus are frequently epidemic in children 5 years of age and younger. Because the clinical presentation of these 2 illnesses in children can be similar, it is useful to distinguish between the two in the seriously ill child. The rapid enzyme immunoassay test which can detect antigens of either influenza A or B directly in respiratory secretions was positive for influenza A virus.  In addition to influenza A virus (the most common and most likely to cause severe disease), infection can occur with influenza B virus (also common) and with influenza C virus (less common, less sever, and does not occur in epidemics). Influenza viruses are single-stranded, segmented, negative sense RNA viruses.
Influenza viruses have two well characterized virulence factors on the surface of the virus, neuraminidase and hemagglutinin. Neuraminidase mediate virus penetration through the mucus layer overlaying the surface of the respiratory epithelium, and it also plays a role in the release and spread of the virus from infected respiratory cells.
Once the virus penetrates to the surface of the cells, binding to specific sialic acid reach receptors is mediated by hemagglutinin. Also, hemagglutinin plays a role in the formation of channels through which viral RNA can enter the cytoplasm and initiate the viral replicative cycle.