System Theory

Learn how focusing on the whole food safety system helps to identify how (contributing factors) and why (environmental antecedents) illness and disease outbreaks happen.

Determining how and why a foodborne illness outbreak happens can be challenging because of the many steps food takes from its original source to the place where it is eaten. At any step along the path, something could happen to introduce contamination to the food, which could trigger an outbreak.

The path that food takes is a system, and each stop along the path are also unique systems (for example, the restaurant system). System theory focuses on understanding a system as a whole along with the underlying interactions of all the forces that make up that system, rather than dissecting a complex process and studying the individual parts. In system theory, it is important to remember the whole is more than the sum of its parts, and a change in one part of the system may affect the other parts or the whole.

Inputs are subjected to processes influenced by internal system variables like people, processes, equipment, foods, and economics. Then those outputs lead to outcomes and External Feedback to the system.

In general, systems are usually made up of the following elements:

  • Input: the energy or material that goes into the system
  • Processes: what happens within the system that changes the input
  • Internal system variables: factors that exert positive, negative, and neutral effects on all other aspects of the system
  • Output: what results from the processes
  • Outcome: what happens as a result of the outputs
  • Feedback: information that can be used to evaluate and monitor the system
The farm-to-fork continuum is the path food takes from source to final service.

This path is a system, and each step along the path is also a unique system. In terms of food safety, the major steps along the farm-to-fork continuum are source, processing and manufacturing, distribution, and point of final service. Each step along the path are unique systems, and outbreak investigators can look at the systems to understand how and why an outbreak happened.


Where the food item originates, including

  • A farm where produce is grown,
  • A body of water where fish are harvested, and
  • A dairy farm or beef cattle operation.
Processing/ manufacturing

All the steps that prepare a food item for distribution. With produce, this includes everything from washing and preparing it for sale to pasteurization or low acidity canning.


Everything from storage and warehousing, repacking, reprocessing, and transport to the next point in the continuum. Sometimes distribution involves multiple points.

Point of final service

Every place the food is purchased and/or eaten, such as delis, cafeterias, restaurants, and grocery stores

Looking at a restaurant as a system can show how each step influences the outcome.
A restaurant system where inputs are ingredients, chemicals, germs; Processes are storing, preparing, cooking, plating, and serving. Internal variables are people, equipment, processes, foods, and economics. Output is final food product served. The Outcome includes customer health and satisfaction. And the outcome provides external feedback to the system.

A restaurant is an example of a system. Each step along the system’s path can influence the outcome of the system (customer health):

  • Inputs, such as ingredients, organisms, chemicals;
  • Processes, such as storing, preparing, cooking, and serving;
  • Internal system variables, such as food workers, equipment, and the economics also influence the outcome;
  • Output, the final food item that is served to the customer;
  • Outcome, such as customer health and satisfaction, profit gain or loss; and
  • Feedback can help inform how processes and variables should change.
Using system theory can help investigators understand how and why a foodborne illness outbreak happened.

In an outbreak response, the sooner investigators identify how and why the germ got into the food, the sooner they can make recommendations on how to stop it from happening (interventions). Looking at the restaurant as a system helps to identify how the outbreak happened (contributing factors) and the root causes of why it happened (environmental antecedent).

For example, a norovirus outbreak might have happened because a food worker was sick (contributing factor). The investigator would try to determine why (environmental antecedents) the worker was working while sick with norovirus. Those reasons could include the following:

  • The worker would not get paid if he didn’t work.
  • His manager said she could not find anyone to work his shift.

After determining how and why the outbreak happened, the investigator can make informed recommendations to the restaurant for preventing another outbreak. The investigator might recommend that the restaurant

  • Provide opportunities for those who miss work because they were sick to make up their hours.
  • Train workers on the importance of not working while sick.
  • Schedule an on-call worker to be available in case another worker is sick.

Identifying contributing factors and environmental antecedents to outbreaks often focuses on inputs, processes, and internal system variables.

Learn more about system theory.

Read about how the systems approach has been used to investigate foodborne and waterborne outbreaks:

The Environmental Health Specialists Network (EHS-Net) uses system theory to better understand the environmental causes of foodborne illness.