Chain of Infection

The chain of infection includes the three factors (tree links) that lead to infection:

1) the reservoir + the etiologic agent;

2) the method of transmission;

3) the host.

These links should be characterized before control and prevention measures are proposed. 

The reservoir of an organism is the site where it resides, metabolizes, and multiplies.

According to the source of infection distinguish:

1. Anthroponoses - people (sick and carriers).

2. Zoonoses - animals.

3. Zooanthroponosis.

4. Sapronoses - objects of the environment.

A carrier is an individual with no overt disease who harbors infectious organisms.

The etiologic agent may be any microorganism that can cause infection. The pathogenicity of an agent is its ability to cause disease; pathogenicity is further characterized by describing the organism's virulence and invasiveness. Virulence refers to the severity of infection, which can be expressed by describing the morbidity (incidence of disease) and mortality (death rate) of the infection. An example of a highly virulent organism is Yersinia pestis,the agent of plague, which almost always causes severe disease in the susceptible host.

The invasiveness of an organism refers to its ability to invade tissue. Vibrio cholerae organisms are noninvasive, causing symptoms by releasing into the intestinal canal an exotoxin that acts on the tissues. In contrast, Shigella organisms in the intestinal canal are invasive and migrate into the tissue.

No microorganism is assuredly avirulent. An organism may have very low virulence, but if the host is highly susceptible, as when therapeutically immunosuppressed, infection with that organism may cause disease. For example, the poliomyelitis virus used in oral polio vaccine is highly attenuated and thus has low virulence, but in some highly susceptible individuals it may cause paralytic disease.

The method of transmission is the means by which the agent goes from the source to the host. 

The transmission routes - the set of environmental factors (air, water, soil, food, household items) that transmit the pathogen from the source of infection to a healthy person.

The method of transmission:

1. Aspiration (airborne transmission) : air-drip and air-dust ways. Droplet spread refers to organisms that travel through the air very short distances, that is, less than 3 feet from a source to a host. Therefore, the organisms are not airborne in the true sense. An example of a disease that may be spread by droplets is measles.

2. Fecal-oral: water, food, contact-household ways. Examples include food-borne salmonellosis, waterborne shigellosis.

3. Transmissible (vector borne transmission) - through blood-sucking arthropods. The method of transmission is vector borne transmission, in which arthropods are the vectors. Vector transmission may be external or internal. External, or mechanical, transmission occurs when organisms are carried mechanically on the vector (for example, Salmonella organisms that contaminate the legs of flies). Internal transmission occurs when the organisms are carried within the vector. If the pathogen is not changed by its carriage within the vector, the carriage is called harborage (as when a flea ingests plague bacilli from an infected individual or animal and contaminates a susceptible host when it feeds again; the organism is not changed while in the flea). The other form of internal transmission is called biologic. In this form, the organism is changed biologically during its passage through the vector (for example, malaria parasites in the mosquito vector).

4. Contact - direct (mycoses) and indirect (contact-household way). Direct contact transmission takes place when organisms are transmitted directly from the source to the susceptible host without involving an intermediate object.  Indirect transmission occurs when the organisms are transmitted from a source, either animate or inanimate, to a host by means of an inanimate object.

5.Vertical transmission mechanism (mother-to-child transmission) :

from the woman to the fetus during pregnancy (through the placenta - the transplacental way), during labor, while breastfeeding the baby (with the mother's milk): chlamydia, rubella, measles, toxoplasmosis, HIV.

• 6. Artificial mechanism: at medical manipulations with the violation of a skin, mucous membranes, through drug addicts syringes that contain the remains of contaminated blood (serum hepatitis, HIV).

An infectious agent may be transmitted by more than one route. For example,  Francisella tularensis may be transmitted by any of the four main (1,2,3,4) routes.

The third link in the chain of infection is the host. The organism may enter the host through the skin, mucous membranes, lungs, gastrointestinal tract, or genitourinary tract, and it may enter fetuses through the placenta. The resulting disease often reflects the point of entrance, but not always: meningococci that enter the host through the mucous membranes may nonetheless cause meningitis. Development of disease in a host reflects agent characteristics (see above) and is influenced by host defense mechanisms, which may be nonspecific or specific.

Nonspecific defense mechanisms include the skin, mucous membranes, secretions, excretions, enzymes, the inflammatory response, genetic factors, hormones, nutrition, behavioral patterns, and the presence of other diseases. Specific defense mechanisms or immunity may be natural, resulting from exposure to the infectious agent, or artificial, resulting from active or passive immunization.

The environment can affect any link in the chain of infection. Temperature can assist or inhibit multiplication of organisms at their reservoir; air velocity can assist the airborne movement of droplet nuclei; low humidity can damage mucous membranes; and ultraviolet radiation can kill the microorganisms. In any investigation of disease, it is important to evaluate the effect of environmental factors. It should be apparent that the occurrence of disease results from the interaction of many factors.

By degree of intensity, the epidemic process is divided into:

Sporadic incidence - single cases - less than 10 per 100000.

Epidemic - the incidence of large masses of the population in large areas (city, region, country), sharply exceeds the level of sporadic morbidity.

Pandemic - the highest incidence rate, covers countries and continents, sharply exceeds the level of epidemic.

The prevalence of infection is divided: 

1. Ubiquitary - widespread.

2. Endemic - diseases that are constantly detected in a certain area:

- natural focal - are associated with natural conditions and the distribution area of ​​the vector and reservoirs;

-statistical - are associated with a complex of climatic and socio-economic factors (cholera in India).

The most dangerous infections:

Convention (quarantine) - are rapidly spreading, highly dangerous to health, international rules have been adopted for elimination these infections (plague, yellow fever, cholera).

Especially dangerous infections are characterized by sudden appearance and rapid spread, severe course, high mortality (anthrax, tularemia, brucellosis, malaria, typhus, poliomyelitis, haemorrhagic fevers).