Infection is a process caused by the entry and multiplication of microorganisms in the tissues of a host. Infectious disease is the manifestation of the infection.

According to the type of microorganism, the infection can be: bacterial, viral, fungal, from protozoa. When the infectious agent comes from the external environment, we speak of exogenous infection; when, on the other hand, it is already present in the organism (on the skin, in the oral cavity, in the gastrointestinal or genitourinary tract) as a harmless commensal (host) before becoming harmful, we speak of endogenous infection.

Microorganisms can penetrate the organism in different ways and modalities. If they are not immediately neutralized by the organism's defenses (immune system), some microorganisms can survive without causing damage in a so-called state of latency (subclinical infection) or, after an incubation period in which they establish and multiply, they can cause an infection that develops rapidly (acute) or, if it does not heal completely, becomes long-lasting (chronic).

The infection can remain localized at the entry point of the microorganism or spread to another organ; it can pass into the blood temporarily (bacteremia) or cause severe impairment of general health (sepsis); in some cases, it can even cross the blood brain barrier and cause meningitis.

The body's defense mechanisms are many. The first barriers to the penetration of germs are the anatomical ones, such as the skin and mucous membranes which, with the sebum and mucus, perform an important antimicrobial activity.

When the microorganisms have already penetrated into the tissues, the organism activates other mechanisms such as:

  • raised body temperature or fever which has defense functions for example against some viruses
  • phagocytosis, a mechanism by which particular cells (phagocytes) incorporate infectious agents and neutralize them
  • inflammatory reaction with activation of the cells that produce cytokines which are mediators of inflammation that act at a local or general level

These functions are part of the so-called innate, or non-specific, immunity, because it is carried out independently of the nature of the infectious agent. If the microorganism survives this type of reaction of the organism, a second level response, or specific immunity, follows, based on the production of antibodies. This type of response takes time to develop.



The disorders (symptoms) caused by an infection are closely related to the organs affected and, despite the diversity of microorganisms, can be similar.

In local infections, inflammation of the affected region (for example, the skin) with redness and swelling may occur. High fever is generally present in acute infections while in chronic forms the fever is low or absent. The body's immune response to an infection can manifest itself with enlargement of local or regional lymph nodes, liver and spleen.

The individual in good health is able to overcome, without any treatment, the most common infections such as, for example, simple skin infections, flu, gastroenteritis but if no improvement is found within a few days, it is well contact your doctor.



Only some microorganisms are pathogens, that is to say capable of producing a disease but, in some circumstances, even the germs that make up the microbial population normally resident in the organism (commensal microorganisms) can cause infections.

The degree of individual susceptibility to infections is influenced by age, nutrition conditions, general health, any treatments that can weaken the body's defense system (immune system).

Most infections are transmitted from person to person (from sick or healthy carriers) but can also be transmitted from infected animals (in this case we speak of zoonosis) or from the environment. The ability of microorganisms to survive in the external environment.

Transmission can take place for:

  • direct contact, for example through sexual intercourse, infected blood, bites, contaminated wounds
  • indirect contact, due to the presence of germs in the droplets of saliva, or in the air (this transmission occurs mainly in closed environments), in water, in food or by transmission through insects
  • congenital transmission, by direct passage, through the placenta, from the mother to the fetus

The development of the infection depends on the characteristics of the microorganisms that are responsible for it: ability to cause infections (infectivity), to develop disease (virulence), to damage tissues (pathogenicity) even with the release of specific toxins (toxicity).

In hospitals, there are many situations favorable to the development of infections (hospital infections): the fragile health conditions of hospitalized people, the use of drugs that weaken the immune system (immunosuppressants), exposure to bacteria present in the environment hospital, often resistant to antibiotics.



The therapy to be used to treat infections is established by the doctor based on the severity of the ailments and the nature of the infectious agent (read the Hoax).

Bacterial infections are treated with antibiotics, locally or generally as needed. Unfortunately, the not always appropriate use of antibiotics and the extreme adaptability of bacteria have contributed to making many of them resistant to various antibiotics.

Antibiotics are not effective against viral infections (such as the flu or cold), so they should not be used for their treatment unless the doctor suspects a simultaneous bacterial infection (superinfection).

Antiviral drugs available to treat viral infections are few and very specific, i.e. they are active only against certain types of viruses. They do not act by directly killing the virus but by blocking its reproduction; their effectiveness may fail because some types of viruses are capable of mutating rapidly.



Prevention plays a decisive role in the control of infections, especially in cases where drug therapy cannot be used (antibiotic-resistant infections, many viral infections, emerging infections).

Vaccines, when available, are a powerful weapon against viruses and bacteria, reducing the appearance of infectious diseases that can be fatal or that can have serious consequences over time.

The use of valid hygiene rules (especially hand washing) and correct behaviors and lifestyles also come into play in the prevention of infections.

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Please refer to the specific infection items.

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