The chain of infection, if we think of it as an actual chain, is made up of six different links: pathogen (infectious agent), reservoir, portal of exit, means of transmission, portal of entry, and the new host. Each link has a unique role in the chain, and each can be interrupted, or broken, through various means. While disease criteria simply define the various ways and means by which disease are cause and spread in the environment.
More specifically, transmission occurs when the agent leaves its reservoir or host through a portal of exit, is conveyed by some mode of transmission, and enters through an appropriate portal of entry to infect a susceptible host. This sequence is sometimes called the chain of infection. The Six Links

The first link is the pathogen itself. This is the disease-causing organism. For many illnesses and diseases this is a virus or bacterium. In order to break this link, various methods can be used, including the pasteurization of milk, the chlorination of drinking water, or the use of disinfectants.
The second link is the reservoir. This is the natural environment that the pathogen requires for survival. Reservoirs can be a person, an animal, or an environmental component, such as soil or water. This link can be broken through medical treatment and testing, insect and rodent eradication, or quarantine.
The third link is the portal of exit. This link is needed for the pathogen to leave the reservoir. If the reservoir is a human, then the portal of exit may be saliva, mucous membranes, feces, blood, or nose or throat discharges. By using barrier methods, such as condoms or masks, or covering the mouth while coughing, this link can be broken.
The fourth link is the means of transmission. The pathogen can be transmitted either directly or indirectly. Direct transmission requires close association with the infected host but not necessarily physical contact. Indirect transmission requires a vector, such as an animal or insect. The link can be broken through hand washing, safe sex practices, or avoiding contact with infected individuals.
Link number five is the portal of entry. Entry of the pathogen can take place in one of three ways: penetration, inhalation, or ingestion. The level and severity of an infection may depend on the depth of penetration. Similar to the portal of exit, barrier methods, such as condoms or masks, can be used to break this link along with other methods, such as insect repellants.
The final link is the new host. Once in the new host, various factors influence the severity of infection, including the strength of the immune system and the reproductive rate of the pathogen. Immunization, health promotion, and medical treatment can be used to break this link in the chain.
Example of a Chain of Infection
An example of illness resulting from the chain of infection is the common cold. In this case, the pathogen is often referred to as rhinovirus. The reservoir is another person carrying this virus, who then propels the virus into the air via a portal of exit, such as a cough or sneeze. The route of transmission is direct to the new host, which takes place through inhalation (the portal of entry) of the virus.
Chain Of Infection – Infection Prevention And Control
Certain conditions must be met in order for a microbe or infectious disease to be spread from person to person. This process, called the chain of infection, can only occur when all six links in the chain are intact.
Infection Control principles are aimed at breaking one or more links in this chain.
• Caustive Agent – the microorganism (for example bacteria, virus or fungi).
• Reservoir (source) – a host which allows the microorganism to live, and possibly grow, and multiply. Humans, animals and the environment can all be reservoirs for microorganisms.
• Portal of Exit – a path for the microorganism to escape from the host. The blood, respiratory tract, skin and mucous membranes, genitourinary tract, gastrointestinal tract, and transplacental route from mother to her unborn infant are some examples.
• Mode of Transmission – since microorganisms cannot travel on their own; they require a vehicle to carry them to other people and places.
• Portal of Entry – a path for the microorganism to get into a new host, similar to the portal of exit.
• Susceptible Host – a person susceptible to the microorganism.
A critical premise of epidemiology is that disease and other health events do not occur randomly in a population, but are more likely to occur in some members of the population than others because of risk factors that may not be distributed randomly in the population. As noted earlier, one important use of epidemiology is to identify the factors that place some members at greater risk than others.
A number of models of disease causation have been proposed. Among the simplest of these is the epidemiologic triad or triangle, the traditional model for infectious disease. The triad consists of an external agent, a susceptible host, and an environment that brings the host and agent together. In this model, disease results from the interaction between the agent and the susceptible host in an environment that supports transmission of the agent from a source to that host. Agent, host, and environmental factors interrelate in a variety of complex ways to produce disease. Different diseases require different balances and interactions of these three components. Development of appropriate, practical, and effective public health measures to control or prevent disease usually requires assessment of all three components and their interactions.
Figure 1.16 Epidemiologic Triad

Image Description
Agent originally referred to an infectious microorganism or pathogen: a virus, bacterium, parasite, or other microbe. Generally, the agent must be present for disease to occur; however, presence of that agent alone is not always sufficient to cause disease. A variety of factors influence whether exposure to an organism will result in disease, including the organism’s pathogenicity (ability to cause disease) and dose.
Over time, the concept of agent has been broadened to include chemical and physical causes of disease or injury. These include chemical contaminants (such as the L-tryptophan contaminant responsible for eosinophilia-myalgia syndrome), as well as physical forces (such as repetitive mechanical forces associated with carpal tunnel syndrome). While the epidemiologic triad serves as a useful model for many diseases, it has proven inadequate for cardiovascular disease, cancer, and other diseases that appear to have multiple contributing causes without a single necessary one.
Host refers to the human who can get the disease. A variety of factors intrinsic to the host, sometimes called risk factors, can influence an individual’s exposure, susceptibility, or response to a causative agent. Opportunities for exposure are often influenced by behaviors such as sexual practices, hygiene, and other personal choices as well as by age and sex. Susceptibility and response to an agent are influenced by factors such as genetic composition, nutritional and immunologic status, anatomic structure, presence of disease or medications, and psychological makeup.
Environment refers to extrinsic factors that affect the agent and the opportunity for exposure. Environmental factors include physical factors such as geology and climate, biologic factors such as insects that transmit the agent, and socioeconomic factors such as crowding, sanitation, and the availability of health services.
Component causes and causal pies
Because the agent-host-environment model did not work well for many non-infectious diseases, several other models that attempt to account for the multifactorial nature of causation have been proposed. One such model was proposed by Rothman in 1976, and has come to be known as the Causal Pies. An individual factor that contributes to cause disease is shown as a piece of a pie. After all the pieces of a pie fall into place, the pie is complete — and disease occurs. The individual factors are called component causes. The complete pie, which might be considered a causal pathway, is called a sufficient cause. A disease may have more than one sufficient cause, with each sufficient cause being composed of several component causes that may or may not overlap. A component that appears in every pie or pathway is called a necessary cause, because without it, disease does not occur. Note in Figure 1.17 that component cause A is a necessary cause because it appears in every pie.

Rothman’s Causal Pies

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Source: Rothman KJ. Causes. Am J Epidemiol 1976;104:587–592.
The component causes may include intrinsic host factors as well as the agent and the environmental factors of the agent-host-environment triad. A single component cause is rarely a sufficient cause by itself. For example, even exposure to a highly infectious agent such as measles virus does not invariably result in measles disease. Host susceptibility and other host factors also may play a role.
At the other extreme, an agent that is usually harmless in healthy persons may cause devastating disease under different conditions. Pneumocystis carinii is an organism that harmlessly colonizes the respiratory tract of some healthy persons, but can cause potentially lethal pneumonia in persons whose immune systems have been weakened by human immunodeficiency virus (HIV). Presence of Pneumocystis carinii organisms is therefore a necessary but not sufficient cause of pneumocystis pneumonia. In Figure 1.17, it would be represented by component cause A.
As the model indicates, a particular disease may result from a variety of different sufficient causes or pathways. For example, lung cancer may result from a sufficient cause that includes smoking as a component cause. Smoking is not a sufficient cause by itself, however, because not all smokers develop lung cancer. Neither is smoking a necessary cause, because a small fraction of lung cancer victims have never smoked. Suppose Component Cause B is smoking and Component Cause C is asbestos. Sufficient Cause I includes both smoking (B) and asbestos (C). Sufficient Cause II includes smoking without asbestos, and Sufficient Cause III includes asbestos without smoking. But because lung cancer can develop in persons who have never been exposed to either smoking or asbestos, a proper model for lung cancer would have to show at least one more Sufficient Cause Pie that does not include either component B or component C.
Note that public health action does not depend on the identification of every component cause. Disease prevention can be accomplished by blocking any single component of a sufficient cause, at least through that pathway. For example, elimination of smoking (component B) would prevent lung cancer from sufficient causes I and II, although some lung cancer would still occur through sufficient cause III.

Unlike infectious diseases of the past, diseases prevalent in modern industrialized societies have multifactorial origins whose complexity so far has defied an integrated scientific understanding. Their epidemiologic investigation suffers from the conceptual inability of formulating plausible causal hypotheses that mimic a complex reality, and from the practical difficulties of running elaborate studies controlled for multifactorial confounders. Until biomedical research provides a satisfactory understanding of the complex mechanistic determinants of such diseases, epidemiology can only field reductionist causal hypotheses, leading to results of uncertain significance. Consensual but rationally weak criteria devised to extract inferences of causality from such results confirm the generic inadequacy of epidemiology in this area, and are unable to provide definitive scientific support to the perceived mandate for public health action

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