Stages of infection describe what happens to a susceptible host once a pathogen invades the human body. The stages of infection are incubation, prodromal, illness, stage of decline, and convalescence. The incubation stage is the period from exposure to a pathogen until symptoms start. The infected person is unaware of impending illness as the pathogens grow and multiply within the body. The duration may vary depending on the type of infection. The incubation period of measles averages ten to twelve days.

During the prodromal stage, the pathogen continues replicating, which activates the body's immune response, allowing mild, nonspecific symptoms to appear. This stage lasts from several hours to several days.

In the stage of illness, the infected person shows noticeable symptoms of infectious disease. The symptoms may be localized or systemic. Localized symptoms affect only one body area, for example, an abscess on the leg. Whereas systemic symptoms affect the entire body and are commonly observed with infections, systemic symptoms affect the entire body.

The type of infection specifies the illness's length and the manifestations' severity. The immune system fights infections after an illness, and symptoms usually improve gradually. However, secondary infections may occur if the primary infection has compromised a person's immune system. For instance, if strep throat spreads throughout the body and infects the patient's heart valves, the heart valves may never fully recover, and heart failure may set in.

Finally, during the convalescent period, the patient regains their health, although some diseases may cause permanent damage.