There is no lack of worry this season over the flu, both the seasonal and H1N1 varieties, but there is a critical lack of understanding of the viruses that cause these illnesses. For years, scientists have recognized that interactions between the influenza virus and its human host — intricate dances involving the virus's genes and proteins and those in humans — are important in determining the course and severity of disease. But a deep, comprehensive knowledge of such host-virus interactions has been elusive.