Human beings differ widely in their responses to virus infections. Viruses may induce barely discernible symptoms in some patients and severe, life-threatening illness in others. These differences reflect both the complex, intimate, and multifaceted nature of virus-host interactions and the variables that can influence these interactions. Viruses elicit a complex temporal sequence of stringently regulated innate and adaptive immune responses. These responses are essential for recovery, but they also contribute to disease symptoms.