Airborne transmission is one of several modes of transmission for infectious agents. It can occur from an environmental source, as in the case of Legionnaires' disease. Under specific conditions, it can occur from a human source. For a given agent, transmission can occur through multiple modes. This is the case, for example, with influenza, which is transmitted both through the air—directly via respiratory secretions (“droplets”)—and indirectly through environmental contamination. While some airborne infections can be transmitted from person to person (such as tuberculosis), they are most often transmitted from a common source involving the environment.