Which term describes the ability of the flu to spread from one individual to another?

The term that accurately describes the ability of the flu to spread from one individual to another is infectivity. Infectivity refers specifically to the capacity of an infectious agent, like the influenza virus, to establish infection in a host and subsequently transmit to others. This characteristic is essential in understanding how contagious a disease is and plays a crucial role in the dynamics of outbreaks and public health responses.

Virulence, while related to the severity of disease caused by an infectious agent, does not specifically address the transmission aspect. Pathogenicity refers to the ability of a pathogen to cause disease, but it also does not capture the transmissibility between individuals. Endemicity describes the constant presence of a disease within a geographic area or population but similarly does not directly pertain to the capacity for spread. Hence, infectivity is the correct term that specifically addresses the spread of influenza among individuals.