Infectious disease

An infectious disease caused by transmission of a micro-organism: viruses, bacteria, parasite, fungus, yeast. Viruses are not alive, but, as the prion, which is not a micro-organism they have properties of type infectious, and therefore pathological effects.

The study of infectious agents is the medicine, epidemiology and the epidemiology. In the wild, infectious diseases are growing among all living organisms (animals, plants, fungi, micro-organisms .. and there is virus virus). As long interaction, infectious diseases are among the feedback loops that maintain the relative stability (dynamic equilibrium) ecosystems, most pathogens co-evolved with their hosts over millions of years. Their mode of transmission is variable and depends on their tanks (human, animal, environmental) and sometimes of vectors (disease vector).

They are more or less contagious; For example, tetanus is an infection caused by Clostridium tetani, a bacterium that is found in the earth. There is no transmission, infection occurs when bacteria enters the body through a wound contaminated. A vaccine exists against the disease and in France is compulsory for all children of school age. Another example, malaria is caused by a parasite, Plasmodium falciparum (there are other Plasmodii), transmitted to humans through a mosquito, Anopheles. The reservoir of the parasite is human, but there is no transmission. There is no vaccine. Tuberculosis is spread from person to person by airborne mechanism: the reservoir is human and it is a contagious disease. Sexually transmitted infections (STIs or STDs for sexually transmitted diseases) are transmitted in connection with sex.

Many microbes live normally and necessarily in our digestive tract and on our skin and become infectious at certain occasions. Contact with microbes is necessary for the maintenance and functioning of the immune system.



The infection is a term to indicate an infectious disease in general, the contamination by a germ. It is the pathological result in a tissue or a body of the abnormal and / or replication of a bacterial germ, viral or fungal.

Contamination is the penetration of the germ in a body.

The infection is the branch of medicine concerning infectious diseases. The doctor is an infectious diseases specialist. Depending on the type of germ, one also speaks of bacteriology, virology, parasitology.

Sepsis is a serious infection. The septic adjective refers to a body or an object contaminated by a germ (eg septic tank). Sepsis is a serious contamination and sustainable (without treatment) from the blood by a germ. Bacteremia is transient contamination of blood by a germ.

When cases are multiplying in a place and a limited period, we speak of an epidemic. If the distribution is much wider, one speaks of a pandemic.

When the epidemic animal middle concerned, there is talk of an outbreak.

When the germ is transmitted from animals to man is spoken of anthropozoonosis or simply zoonosis.

The infection means the contamination by the germ.

The incubation period is the period between infection and the first manifestation of the disease. The patient may be contagious during this time.

The period of contagion is the time during which the patient and excrete the seeds can pass. It depends on each disease.

Nosocomial infections are infections caught in hospital. They are particularly complex and dangerous because they occur in people and often weakened germs resistant to antibiotics. This is a major public health problem.


In the world, infectious diseases are responsible for 17 million deaths per year, representing one third of mortality. They represent 43% of deaths in developing countries, against 1% in industrialized countries.

The following six diseases account for 90% of deaths from infectious diseases in the world.
Disease Number of deaths per year
in the world
Respiratory Diseases 3 million
AIDS 3 million (2001)
Diarrheal diseases 2.5 million
Almost 2 TB millions1
Malaria more than 1 million deaths
Measles 900 000

Also note that certain infections are also the cause of chronic inflammatory diseases (such as asthma) and cancer.

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