The time between primary and secondary case is called:

Serial interval - gap in time between onset of the primary case and the secondary case. A period of communicability - time during which an infectious agent may be transferred directly or indirectly from an infected person to another person, from an infected animal to man or from an infected person to an animal including ahropods. Incubation period - time interval between invasion by an infectious agent and appearance of the first sign or symptoms of the disease Generation time - interval of time between reciept of infection by a host and maximal infectivity of the host Ref: Park 21st edition, page 95.