For instance, transmission of respiratory syncytial virus (RSV) appears to be mediated mainly by direct and indirect contact, while severe acute respiratory syndrome coronavirus spread via airborne routes, including droplet spray and aerosol, seemed to play the prominent role in its dissemination. Although human respiratory diseases of viral origin can be virtually indistinguishable clinically, the causative viruses are quite heterogeneous, not only in their virion structure and genome composition but also in the routes by which they transmit among human. Many theories have been proposed to explain the specific effect of environmental variables on the pronounced seasonality of influenza and, to a lesser extent, disease caused by RSV and “common cold” viruses. The seasonality of other respiratory viruses has been much less studied than that of influenza or RSV. The data on adenovirus circulation is scant, so overall patterns of seasonality are difficult to discern; however, it is often said to transmit primarily in the winter or early spring in temperate climates.