Influenza is the best known model of bacterial-viral co-infection. Epidemics of influenza result in an increased hospital admission rate for bacterial pneumonia due to pneumococcus, Haemophilus influenzae and Staphylococcus aureus. Similarly, an increased incidence of meningococcal diseases, particularly severe forms, follows the influenza outbreaks, with a two week delay. Though the precise mechanism is not known, the depression of host's phagocytes bactericidal activity by the influenza virus seems to be involved. An increased incidence of invasive group A beta hemolytic streptococcal infections, particularly necrotizing fasciitis and toxic shock syndrome, is also observed in relation with chickenpox. The reason for this association is unclear and appears not to be limited to the disruption of the cutaneous barrier which leads to the cutaneous infections in this disease. Bacterial-viral co-infection is not a justification for a systematic antibiotic prescription in viral diseases. Severe bacterial disease will be best prevented through viral immunization, thus encouraging the development of viral vaccines and immunization campaigns.