Viruses are among the smallest infectious particles that can cause diseases in humans. Viruses are typically introduced into the body through close contact, respiration, transfusions, or sexual transmission. Although some viruses can withstand harsh environmental conditions, they must enter a host cell to successfully propagate. Due to their compact size, they do not possess the complete machinery to replicate. They thus employ the components of the host cell for this and other functions.

The diseases that viruses cause can either result from direct cellular injury or indirectly through the body's response to their presence. Literally billions of people are annually infected with disease-producing viruses, which results in an enormous impact on the global economy.