Respiratory tract infections are the most common infectious illnesses that afflict humans. In general, viral infections of the upper respiratory tract result in mild self-limiting symptoms. However, more serious lower respiratory tract infections can lead to the development of pneumonia. While viral infections alone may cause pneumonia, it is increasingly clear that they also play an important role in increasing the risk of bacterial infection and have been shown to substantially worsen clinical outcomes from bacterial pneumonia. Advances in our understanding of the immune response to these infections are beginning to demonstrate how these microorganisms can interact, subvert the normally effective immune response and facilitate the development of more severe and even life-threatening disease.