Medical biology, microbiology, virology, immunology department

by As. Prof. O.V. Pokryshko

The interaction of the susceptible human organism and the pathogenic microorganism in certain conditions of the external and social environment which gives rise to an obvious or latent pathological process is called an infectious process.

The term infection (L. infectio to infect) signifies the sum of biological processes which take place in the macroorganism upon the penetration of pathogenic microorganisms into it.

Infectious disease designates one of the extreme degrees of manifestation of the infectious process.

Infectious diseases are considered to be phenomena including biological and social factors.

Infectious diseases differ from other diseases in that

they are caused by live causative agents

of a plant and animal origin and are contagious, the presence of a latent period, specific reactions of the body to the causative agent and production of immunity

For the origination and development of the infectious process three conditions are necessary: 1. the presence of pathogenic microbe,2. its penetration in to a susceptible macroorganism, 3. certain environmental conditions in which the interaction between the microorganism and macroorganism takes place.

Koch’s postulates.

What is an Epidemic?What is an Epidemic?The occurrence of more cases of The occurrence of more cases of

disease than expected in a given area or disease than expected in a given area or among a specific group of people over a among a specific group of people over a particular period of time*.particular period of time*.

The occurrence of more cases of The occurrence of more cases of disease than expected in a given area or disease than expected in a given area or among a specific group of people over a among a specific group of people over a particular period of time*.particular period of time*.

EpidemicEpidemic

What is a Pandemic?What is a Pandemic?

An epidemic occurring over a very wide An epidemic occurring over a very wide area (several countries or continents) area (several countries or continents) and usually affecting a large proportion and usually affecting a large proportion of the population.of the population.

Examples:Examples:– CholeraCholera– AIDSAIDS– Pandemic InfluenzaPandemic Influenza

An epidemic occurring over a very wide An epidemic occurring over a very wide area (several countries or continents) area (several countries or continents) and usually affecting a large proportion and usually affecting a large proportion of the population.of the population.

Examples:Examples:– CholeraCholera– AIDSAIDS– Pandemic InfluenzaPandemic Influenza

PandemicPandemic

Source of infectionSource of infection::Sick menSick animalCarrier

Transmission factorTransmission factor:: infected foodinfected food, , waterwater, , dirtydirty handshands, , fliesflies, , fomitesfomites..

VectorsVectors of pathogenicof pathogenic microorganimsmicroorganims: : ticksticks, , louseslouses, , fleasfleas mosquitosmosquitos..

Routes of diseases transmission

There are various modes which pathogens are transmitted from a source to a susceptible individual.

airborne transmissionfoodborne transmissionsexual transmissiondirect contact transmissionparenteral route

Respiratory tract and airborne transmission.

Airborne transmission occurs when pathogenic microorganisms are transferred from an infected to a susceptible individual via the air. Droplets regularly become airborne during normal breathing, but the coughing and sneezing associated with respiratory tract infections are primarily responsible for the spread of pathogens in aerosols and thus the airborne transmission of disease.

High-speed photograph of an aerosol generated High-speed photograph of an aerosol generated by an unstifled sneeze.by an unstifled sneeze.

Gastrointestinal tract—water and foodborne transmission.

Microorganisms routinely enter the gastrointestinal tract in association with ingested food and water. Waterborne and foodborne pathogens can infect the digestive system and cause gastrointestinal symptoms.

Genitourinary tract – sexual transmission.

The genitourinary tract provides the portal of entry for pathogens that are directly transmitted during sexual intercourse. Such infections are known as venereal or sexu- ally transmitted diseases.

Superficial body tissues – direct contact transmission.

In some cases the deposition of pathogenic microorganisms on the skin surface can lead to an infectious disease. Since they require direct contact between skin and microorganisms for transmission to occur, these diseases are called contact diseases. Some diseases transmitted in this manner are superficial skin infections.

Parenteral route. Punctures, injections, bites, cuts, wounds, surgical incisions, and cracking skin due to swelling or drying establish portals of entry to a host for a potential pathogen. Such access is called the parenteral route (from Greek para [beside] and enterik [intestinal tract]. Microorganisms thus gain entry to the body by being deposited directly into the tissues beneath the skin or into the mucous membrane.

Portals of entry. The routes of entry are the respiratory tract, gastrotestinal tract, genitourinary tract, skin, and wounds.

The invasive properties of specific pathogens permit them to penetrate the body's defense mechanisms through a specific portal of entry. Most pathogenic microorganisms will cause disease only if they enter the body via this specific route.

The dynamics of the development of the infectious process consists of

1. the incubation period 2. prodromal period, 3. the height of the disease 4. period of recovery ( convalescence).

A certain period of time elapses from the moment of penetration of the pathogenic microbe to the onset of the first sings of the disease, which has been named the incubation period of the disease.

The sites at which microorga- nisms leave the body are called portals of exit. Pathogens of the genitourinary and gastrointestinal systems generally exit the body with body fluids or feces. Pathogens of the respiratory sys- tem exit through the nose or mouth in fluids expelled during coughing, sneezing, and speaking. Some pathogens, such as the bacteria that cause tuberculosis, are resistant to desiccation and can remain viable in the air for a long while. These organisms can be transmited via the air, and inhalation of contaminated air can lead to infection.