Stages of Infection: Infectious Disease Process

Anne Kamiya, Artem Cheprasov
  • Author
    Anne Kamiya

    Anne has experience in science research and writing. She has a graduate degree in nutrition (gut microbiome & nutritional microbiology) and undergraduate degrees in microbiology (immunology & medical microbiology) and English (myth & folklore). She has also worked as an ocean & Earth science educator.

  • Instructor
    Artem Cheprasov

    Artem has a doctor of veterinary medicine degree.

Understand the stages of infection. Examine the prodromal stage, incubation stage, and convalescent period, and learn about the disease process and latent disease. Updated: 06/07/2022

Table of Contents


Stages of Infection

Sayuri and some of her relatives ate chicken salad at a family gathering. Little did they know that the mayonnaise used to make the chicken salad was contaminated with the bacteria Salmonella enterica. Consuming Salmonella can cause a diarrheal illness called salmonellosis. Sayuri got sick that night, and her cousin got sick the next day, but her brother and two aunties never got sick at all. Not everyone who ingests Salmonella will get sick. For the people who do get sick, it may take hours to days to show symptoms.

An infection is an illness caused by a harmful microscopic organism that invades the body, called a pathogen, such as a bacteria, virus, parasite, or fungi. Various factors can affect whether someone becomes ill from pathogens, like its infectivity (how well it multiplies in the body), pathogenicity (its ability to cause disease), and virulence (how well it causes severe disease). The immune system of the person being infected also plays a huge role in the process of infection. If the immune system is able to fight off the pathogen before it can establish an infection, no signs of disease will occur.

The infectious disease process is categorized into different stages of infection. Infections generally involve incubation, prodromal, illness, decline, and convalescent stages. Stages of infection apply to all types of infectious diseases, not just food-borne illnesses.

Incubation Stage

In infectious diseases, the incubation stage is defined as the period between the first exposure to a pathogen and the first emergence of symptoms. During the incubation stage, a person does not show any signs or symptoms of sickness.

When a pathogen gets inside of the body, one of the first things it does is multiply. A few pathogens are rarely enough to cause illness, so they have to build up their numbers. It is kind of like how it would be futile to attack a castle with just a handful of warriors. The few warriors would be devastated by castle defenses. But, laying low until enough troops are gathered and then approaching the castle with ten thousand warriors would be quite formidable. This analogy describes incubation, where the invader is present and expanding its numbers, but not yet formidable enough to cause symptoms.

Whether or not a person can spread the infection to other people during the incubation stage depends on the pathogen. For example, influenza is not contagious until someone is symptomatic, chickenpox is contagious a few days before someone shows signs of illness, and a person with human immunodeficiency virus (HIV) is contagious shortly after infection, before they even know they are sick.

The duration of incubation varies based on the type of pathogen. Incubation can last hours, days, months, or (rarely) years. Salmonellosis usually makes someone ill within two days. Chickenpox has an incubation period of two to three weeks after exposure. Hepatitis B may take two or three months before symptoms emerge, while someone with HIV may be asymptomatic for several months or years.

Prodromal Stage

The prodromal stage is an intermediate period between incubation and illness. During this stage, pathogens continue to multiply but are still not formidable enough to cause fulminant illness. A person also begins to show signs of illness, but the symptoms are mild and not very specific or diagnostic. Prodromal symptoms vary based on the type of infectious disease but generally indicate early immune activation and include malaise and fever. The duration of time can vary, but the prodromal stage tends to be shorter than the incubation period. A person is contagious and can transmit an infection to others during this phase.

Period of Illness

When a person is highly contagious and experiencing fulminant symptoms, they are facing the period of illness. Microbial replication steadily increases during this stage and includes the peak of infection, called an acme point. Duration of illness and signs and symptoms vary widely based on the pathogen and infectious disease it causes. For example, if someone has influenza, they will experience the full symptoms of a flu, including fever, body aches, cough, and fatigue. If someone has rabies, they will show symptoms of fulminant lyssavirus encephalitis, such as hydrophobia, delirium, agitation, and coma.

Period of Decline

Once the acme point is reached, invading pathogens decrease in the body as the immune system combats the infection during the period of decline. Symptoms start to wane; however, a secondary infection may develop due to stress on the immune system from combating the primary infection. For example, as someone recovers from the flu, which is a self-limiting viral illness, they may develop a secondary bacterial pneumonia.

Convalescent Period

The final stage of infection is called the convalescent period and is typically a period of recovery and return to the pre-illness state. Microbial replication has halted thanks to the immune system, but a person may still be contagious during convalescence, depending on the pathogen. However, not all persons will make it to convalescence or even fully recover.

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  • 0:48 Incubation Period
  • 2:07 Prodromal Period
  • 2:50 Period of Illness
  • 3:45 Period of Decline
  • 4:24 Period of Convalescence
  • 6:32 Lesson Summary
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Frequently Asked Questions

What are the stages of infection?

When someone contracts an acute infection, they will experience five stages of infection:

  1. First comes the incubation period, where a person does not yet feel ill.
  2. Second comes the prodromal stage, where mild symptoms start to appear.
  3. Third comes the period of illness, which is the point of fulminant illness.
  4. Forth comes the period of decline, where a person starts to feel better and recover.
  5. Lastly comes the convalesce period, where full recovery takes place.

What are the types of disease?

There are three types of diseases, which each have different stages of illness. Acute diseases are short-lived and present with incubation, prodromal, illness, decline, and convalescent stages. Chronic diseases are long-term illnesses that do not resolve because the immune system cannot clear the pathogens, leaving a person perpetually stuck in the period of illness. Latent illnesses are similar to acute diseases, except the microorganism, usually a virus, hides in the body as a latent infection after convalescence, reemerging months or years later.

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