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Lecture 17,18

Theory of infection. Pathogenicity and virulence. Factors of virulence.

Types of infection. Elements of epidemiology. Mechanisms and ways of infection transmission.

Infection is the invasion of the host by microorganisms, which then multiply in close association with the host's tissues

Infectious process - the process of interaction m / o and the human body, a complex of physiological and pathological reactions aimed at restoring homeostasis.

Infectious disease - the extreme (the most pronounced) form of the infectious process with the formation of a pathological focus and the emergence of specific clinical symptoms.

Infection does not necessarily lead to infectious disease. In fact, infection
probably rarely leads to infectious disease. Some bacteria rarely cause disease if
they do infect; some bacteria will usually cause disease if they infect. But other
factors, such as the route of entry, the number of infectious bacteria, and status
of the host defenses, play a role in determining the outcome of infection.


1. Invasion of m / o in the body and adhesion(attachment to sensitive cells).

2.Colonization (multiplication on their surface) and penetration (penetration of m / o inside the cells of the macroorganism).

3. Dissemination - the spread of m / o beyond the primary focus of the invasion.

4. Mobilization of the protective factors of the organism


The end and the outcome (sanation and formation of immunity, carrier , host death).

The development of the infectious process depends on:

1) the properties of the pathogen;

2) the state of the macroorganism;

3) from environmental conditions, which can affect both the state of the pathogen and the state of the macroorganism.


1. A pathogenic microbe.

2. A susceptible macroorganism.

3. A favorable environment/

A pathogen is a microorganism (or virus) that is able to produce disease. Bacteria which cause a disease in a compromised host which typically would
not occur in a healthy (noncompromised) host are acting as opportunistic
pathogens. A member of the normal flora can such as Staphylococcus aureus or
E. coli can cause an opportunistic infection, but so can an environmental
organism such as Pseudomonas aeruginosa. When a member of the normal flora
causes an infectious disease, it might be referred to as an endogenous bacterial
disease, referring to a disease brought on by bacteria “from within”.

There are some pathogens that do not associate with their host EXCEPT in the case of
disease. These bacteria are obligate pathogens, even though some may rarely
occur as normal flora, in asymptomatic or recovered carriers, or in some form
where they cannot be eliminated by the host.

Finally, some bacteria, such as Lactobacillus acidophilus, are considered to be nonpathogens, because they rarely or never cause human disease. Their categorization as nonpathogens may change, however, because of the adaptability of bacteria and the detrimental effect of modern radiation therapy, chemotherapy, and immunotherapy on resistance mechanisms. In fact, some bacteria previously considered to be nonpathogens are now known to cause disease. Serratia marcescens, for example, is a common soil bacterium that causes pneumonia, urinary tract infections, and bacteremia in compromised hosts.

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