Primary and secondary case are used to define-

. in actual practice we seldom know precisely the incubation period of a disese.but we know when an outbreak of disease occurs,say in a family of which is the smallest grouop and primary case is followed by 2 or 3 secondary cases within a sho time.the gap in time between the onset of the primary case and the secondary case is called the "serial interval" ref:park&;s textbook of preventive and social medicine,22 nd edition,pg no96