Infectious Disease Process

Stages of Infection: Infectious Disease Process
Stages of Infection
At a family gathering, Sayuri and some of her relatives ate chicken salad. They had no idea the mayonnaise they were using to make the chicken salad was contaminated with the bacteria Salmonella enterica. Salmonellosis is a diarrheal illness caused by Salmonella. Sayuri became ill that night, and her cousin became ill the next day, but her brother and two aunties did not. Not everyone who consumes Salmonella will become ill. It may take hours to days for people who do get sick to show symptoms.

An infection is a disease caused by a pathogen, which is a harmful microscopic organism that invades the body and causes illness. Pathogens include bacteria, viruses, parasites, and fungi. Pathogens’ infectivity (how well they multiply in the body), pathogenicity (their ability to cause disease), and virulence are all factors that can influence whether someone becomes ill from them (how well it causes severe disease). The infected person’s immune system also plays a significant role in the infection process. There will be no signs of disease if the immune system is able to fight off the pathogen before it can establish an infection.

The infectious disease process is divided into stages of infection. In general, infections have incubation, prodromal, illness, decline, and convalescent stages. Infection stages apply to all infectious diseases, not just food-borne illnesses.

Incubation Stage
The incubation stage in infectious diseases is defined as the time between the first exposure to a pathogen and the onset of symptoms. During the incubation period, a person exhibits no signs or symptoms of illness.

One of the first things a pathogen does once inside the body is multiply. A few pathogens are rarely enough to cause illness, so they must multiply. It’s similar to how attacking a castle with only a few warriors would be pointless. Castle defenses would decimate the few warriors. However, hiding until enough troops are gathered and then charging the castle with ten thousand warriors would be quite formidable. This analogy describes incubation, when the invader is present and multiplying but not yet powerful enough to cause symptoms.

The pathogen determines whether or not a person can spread the infection to others during the incubation stage. For example, influenza is not contagious until someone becomes symptomatic, chickenpox is contagious a few days before symptoms appear, and a person infected with human immunodeficiency virus (HIV) is contagious shortly after infection, before they even realize they are sick.

The length of incubation varies depending on the pathogen. Incubation can last hours, days, months, or even years in rare cases. Salmonellosis typically causes illness within two days. Chickenpox takes two to three weeks to develop after exposure. Hepatitis B symptoms may appear two to three months later, whereas HIV may be asymptomatic for months or years.

Prodromal Stage
The prodromal stage is the time between incubation and illness. Pathogens continue to multiply during this stage, but they are not yet powerful enough to cause fulminant illness. A person begins to exhibit symptoms of illness as well, but the symptoms are mild and not very specific or diagnostic. Prodromal symptoms vary depending on the type of infectious disease, but typically include malaise and fever and indicate early immune activation. The time span can vary, but the prodromal stage is usually shorter than the incubation period. During this stage, a person is contagious and can spread an infection to others.

Illness Duration
When a person is highly contagious and suffering from fulminant symptoms, they are in the midst of an illness. During this stage, microbial replication steadily increases, culminating in an infection peak known as an acme point. The duration of illness, as well as the signs and symptoms, vary greatly depending on the pathogen and infectious disease it causes. For example, if someone has influenza, they will have all of the flu symptoms, such as fever, body aches, cough, and fatigue. Rabies causes symptoms of fulminant lyssavirus encephalitis such as hydrophobia, delirium, agitation, and coma.

Period of Illness
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