Which of the following statements about the development of infectious diseases is correct?
a) The period of decline is the time when the infected individual's health rapidly deteriorates.
b) The period of convalescence is the time during which the person regains health and fully recovers (back to the pre-disease state).
c) The prodromal period is characterized by very severe symptoms.
d) During the incubation period, the infected individual exhibits obvious signs of sickness.

Diseases that can spread from one person to other are called infectious disease.
Disease development is divided into Incubation period, Prodromal period, period of illness, Decline period, and period of Convalescence.
During incubation period the pathogen enters the host body and starts multiplying itself. But the signs of diseases are unnoticeable.
Then comes the prodromal period, when the pathogen keeps multiplying itself and some signs of disease are observed but they are not obvious
After the prodromal period comes the period of illness when the patient exhibits obvious signs of sickness.
As the period of illness is over the symptoms of disease start to cease and the the number of pathogen decrease in body.This period is the period of decline.
Finally comes the period of Convalescence when the person returns to his pre-disease state and is completely healthy. (except irreversible damage caused by the disease)