Why is coinfection with influenza virus and bacteria so difficult to control?

Influenza viruses are genetically labile pathogens which avoid immune detection by constantly changing their coat proteins. Most human infections are caused by mildly pathogenic viruses which rarely cause life-threatening disease in healthy people, but some individuals with a weakened immune system can experience severe complications. Widespread infections with highly pathogenic strains of influenza virus are less common, but have the potential to cause enormous death tolls among healthy adults if infection rates reach pandemic proportions. Increased virulence has been attributed to a variety of factors, including enhanced susceptibility to coinfection with common strains of bacteria. The mechanisms that facilitate dual infection are a major focus of current research, as preventative measures are needed to avert future pandemics.