In the context of epidemiology, when a susceptible person develops a disease after coming into contact with the primary case, this is most accurately described by the term "Secondary attack rate."
The Secondary attack rate measures the spread of disease in a defined population after the introduction of an initial case. It is used to identify the probability of infection among susceptible persons after exposure to a primary case. This metric is vital for understanding how rapidly a disease can spread within close-contact environments.
Thus, the correct choice is: