This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
The time interval between invasion of the infection agent and appearance of the first sign or symptom 
Select and explain. 
A. Serial interval, B. Incubation period, C. Quarantine, D. Period of infectivity
B: Incubation period
Answer: B: Incubation period 
Explanation: 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, Park's Textbook of Preventive and Social Medicine, Page no. 107