Latency and Incubation Periods

The starting point of a communicable disease is the exposure of the host to the infectious agent. When communicable diseases are considered as a whole, two different processes have to be distinguished in their evolution over time: infectiveness and disease. Infectiveness consists of two successive periods: the latency period and the period of communicability. The disease process encompasses the incubation period and the clinical signs and symptoms. Whereas the knowledge of infectiveness is of paramount importance for microbiological, pharmacological, and public health purposes, the main interest in the disease process hinges on the clinical care of the patient.

The incubation period is the time that elapses between the initial exposure of the host to the infectious agent and the first appearance of the clinical manifestations (signs or symptoms) associated with the disease. In a vector, it is the time between entrance of an infectious agent into the vector and the time when that vector can transmit the infection (a period known as the extrinsic incubation period).

The duration of the incubation period is determined by the time needed by the infectious agent to grow enough in number in the host to produce symptoms. During the incubation period, the infectious agent can be transferred from one host to another. In many diseases, the communicable period begins before the inception of signs and symptoms, as, for example, in viral hepatitis and some exanthematic infections (those characterized by skin eruptions), such as measles, rubella, scarlet fever, and chickenpox.

The incubation period is known for most diseases. It varies among diseases, and for each particular disease, although in a much lesser degree, depending on the infective dose of the agent, namely, the mean number of microorganisms needed to cause infection. (The infectiveness of an infectious agent is usually expressed as the infectious dose 50 [ID50, and it is defined as the infectious dose needed to produce infection in 50% of the susceptible hosts.) Some examples of incubation periods are as follows: influenza (from 1 to 3 days), diarrhoea caused by Escherichia coli (from 3 to 8 days, with a median of 3 to 4 days), measles (from 7 to 18 days, with a median of 10 days), hepatitis A (from 15 to 50 days, depending on the infective dose, with an average of 28 to 30 days), and leprosy (from 9 months to 20 years, with a probable average of 4 years for tuberculoid leprosy and 8 years for lepromatous leprosy).