Time interval between invasion of the infection agent and appearance of first sign or symptom ?

Ans. is `b' i.e., Incubation period Incubation period o Clinical symptoms of an infectious disease are not produced immediatly after invasion of infectious agent into the host. o First infectious agent multiplies and produces some cellular/tissue response which results in clinical manifestations. So there is an interval between invasion of infectious agent and onset of clinical manifestations which is known as incubation period i.e., "the time interval between invasion by an inectious agent and appearance of the first sign and symptom of the disease in question". o The length of incubation period is characteristic of each disease. o There is minimum incubation period for each disease before which no illness can occur. Incubation period varies for different infection and also from person to person with same disease. It is so because incubation depends upon many factors : - i) Generation time of pathogen iii) Poal of entery ii) Infective dose iv) Individual susceptibility Median incubation period It is the time required for 50% of cases to occur following exposure.