<prompt> The time interval between invasion of the infection agent and appearance of the first sign or symptomSerial intervalIncubation periodQuarantinePeriod of infectivity  <response>: An infection becomes apparent only after a ceain incubation period which is defined as the time interval between invasion by an epidemiological agent and the appearance of first sign or symptoms. During the incubation period, the infectious agent undergoes multiplication in the host. Ref: 25th edition, Parks Textbook of Preventive and Social Medicine, Page no. 107 