Influenza and invasive bacterial infections: Co-infections involving the influenza virus and certain bacteria are frequently observed; intense influenza virus activity can thus contribute to an increased risk of invasive bacterial infections, particularly those of respiratory origin, caused by group A streptococcus, pneumococcus, Staphylococcus aureus, and Haemophilus influenzae. Viral infections also increase the risk of post-influenza invasive meningococcal infections.