Mixed respiratory viral infections in Estonia: a long-term laboratory study.

During the period of 1981-1997, a total of 13,110 patients with acute respiratory viral infections were tested simultaneously for influenza virus A and B, parainfluenza, adenovirus and respiratory syncytial virus (RSV) infections. Of 3,800 laboratory-confirmed cases mixed infections were established in average in 27%. The incidence varied in dependence on the season between 12% and 36%. All infections were involved in mixed infections, yet their participation greatly varied in different seasons. During a season when the incidence of two respiratory viral infections increased, the case number of their mixed infections also increased. We assume that respiratory viruses widely co-circulating among human population also simultaneously infected the same persons. Therefore a proportion of respiratory viral diseases may represent in fact mixed infections which, in turn, may lead to the emergence of mixtures of different viruses and their spread in humans.