Epidemic process

Biosocial nature and the triad of the process

The biological foundation of any epidemic is a parasitic system—the continuous interaction between populations of a microorganism (parasite) and a human (host). This interaction leads to mutual adaptive changes and can manifest in various ways: from latent asymptomatic carriage to manifest, i.e., pronounced, forms of the disease.

For the epidemic process to proceed continuously, three mandatory links must be present simultaneously. Eliminating even one of them completely interrupts the development of the epidemic:

1. Source of infection. This is a living organism or an abiotic (non-living) object that serves as a natural place for the life and reproduction of the pathogen. Based on this, diseases are divided into anthroponoses (the source is exclusively a sick person or carrier), zoonoses (the source is an animal, although humans can also fall ill), and sapronoses (the source is non-living environmental objects).
2. Mechanisms, routes, and factors of transmission. The ways the pathogen is transferred to a new host.
3. Susceptibility of the population. The presence in the human population of individuals lacking protection against the microbe.

Gromashevsky's law and localization of the pathogen

The transmission mechanism is realized through specific routes and factors. According to L.V. Gromashevsky's law, there is a strict correspondence between where the pathogen is localized in the host's organism and the mechanism by which it is transmitted:

• Gastrointestinal tract. Characterized by the fecal-oral mechanism. Routes of transmission: alimentary (via food), waterborne (the factor is water), or contact-household (via dirty hands, shared utensils, and flies).
• Respiratory tract. Operates via the aerogenic (respiratory) mechanism. It includes airborne (via the air itself) and dust-borne (with dust acting as the factor) routes.
• Blood. The pathogen is transmitted via the hematogenous mechanism. This includes transmission through bites of blood-sucking ectoparasites (transmissive route), as well as the parenteral route (via contaminated blood, non-sterile medical instruments, or infusion solutions).
• External integument. The contact mechanism is activated: the wound route (factor: traumatic objects) or contact-sexual route.
• Germ cells. The vertical mechanism is implemented when the pathogen passes from mother to child via the placenta (transplacental transfer).

Quantitative assessment and intensity

To understand the severity of the situation, a quantitative assessment of disease spread is used. For this, intensive morbidity (or mortality) rates are calculated—the number of identified cases per 10,000 or 100,000 population. When calculating, it is mandatory to specify the exact nosology, exact territory, and a defined time period.

Depending on the figures obtained, three degrees of intensity of the epidemic process are distinguished:

• Sporadic morbidity. This is the usual, baseline level for a specific disease in a given territory at a set time.
• Epidemic. A sharp excess of the sporadic morbidity level in the form of a mass outbreak.
• Pandemic. The highest degree of intensity, significantly exceeding the scale of a usual epidemic. It is characterized by the rapid seizure of new territories and global spread.

Infection control and response measures

From a practical standpoint, all infections are classified according to their controllability. If effective means of intervention exist against a disease (e.g., vaccines), it is considered controllable. The strategic goal in this case is the complete global eradication of the infection. If no effective control measures exist, the infection belongs to the group of uncontrollable.

Anti-epidemic measures themselves are strictly grouped depending on which of the three links of the process they target:

• First group (targeting the source): isolation of infected individuals, their treatment.
• Second group (targeting transmission routes): disinfection, compliance with hygiene rules.
• Third group (targeting the population): immunoprophylaxis and vaccination. The main task here is to create an artificial immune layer (the proportion of protected people). If the threshold is 95% and higher, epidemic well-being is achieved.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mutual adaptive changes in the parasitic system
• Specific transmission factors for different mechanisms
• Strategies for the global eradication of controllable infections
• Calculation of intensive morbidity rates

Other topics in the section "Study of Infection"

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