Mixed respiratory viral infections are double negative common and evidence that they are associated with severe disease is supported by some groups. This controversial observation can be explained by the lack of sensitivity of the assessed methods used for viral identification and by the small number of patients included in the randomized cohorts studied. Most studies showed that Respiratory Syncytial Virus (RSV) is identified in about 70% of hospitalized infants with bronchiolitis during seasonal winter epidemics, followed by human metapneumovirus (about 3%-19%) or Rhinoviruses (about 20%). Other respiratory viruses have also been reported, indicating significant causes of bronchiolitis and hospitalization during seasonal epidemics. The presence of more than one pathogen, and moreover, the association of RSV with rhinoviruses and also RSV with hMPV, may influence the natural course of bronchiolitis. A better understanding of these various interactions would help future decision-making, such as the extent to which searches for co-pathogens should be conducted in severe bronchiolitis patients already infected by RSV.