<prompt> An infection becomes apparent only after a ceain incubation period. Shoest incubation period is seen in:Hepatitis AHepatitis BRubellaInfluenza  <response>: Incubation period is defined as the time interval between invasion by an infectious agent and appearance of the first sign or symptom of the disease in question. Incubation period of, Influenza - 18 to 72 hours Hepatitis A - 15 to 45 days usually 25 to 30 days Hepatitis B - 45 to 180 days Rubella German Measles - 2 to 3 weeks; usually 18 days Ref: Parks Textbook of Preventive and Social medicine, 19th edition, Page 93, 134. 