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infection, invasion of plant or animal tissues by microorganisms, i.e., bacteria, viruses, viroids, fungi, rickettsias, and protozoans. The invasion of body tissues by parasitic worms and other higher organisms is commonly referred to as infestation.
Invading organisms such as bacteria produce toxins that damage host tissues and interfere with normal metabolism; some toxins are actually enzymes that, by breaking down host tissues, prevent the localization of infections. Other bacterial substances destroy the host's phagocytes. Viruses and retroviruses are parasitic on host cells, causing cellular degeneration, as in rabies, poliomyelitis, and AIDS, or cellular proliferation, as in warts and cold sores. Some viruses have been associated with the development of certain cancers. Substances produced by many invading organisms cause allergic sensitivity in the host; the immune response to virus infection has been implicated in some diseases (see allergy).
Infections may be spread via respiratory droplets, direct contact, contaminated food, or vectors, such as insects. They can also be transmitted sexually (see sexually transmitted diseases) and from mother to fetus. Immunity is the term used to describe the capacity of the host to respond to infection. Drugs that help fight infections include antibiotics and antiviral drugs.
See also specific diseases, diseases of plants.
See J. Waller, The Discovery of the Germ (2003).
A term considered by some to mean the entrance, growth, and multiplication of a microorganism (pathogen) in the body of a host, resulting in the establishment of a disease process. Others define infection as the presence of a microorganism in host tissues whether or not it evolves into detectable pathologic effects. The host may be a bacterium, plant, animal, or human being, and the infecting agent may be viral, rickettsial, bacterial, fungal, or protozoan.
A differentiation is made between infection and infestation. Infestation is the invasion of a host by higher organisms such as parasitic worms. See Epidemiology, Medical bacteriology, Medical mycology, Medical parasitology, Opportunistic infections, Pathogen, Virus
penetration of a pathogenic parasite into a human or animal organism and the state of being infected. The concept of infection is also applied to one-celled organisms (bacteriophages). On the other hand, there is a tendency to distinguish between the concept of infection and that of parasitism, including invasion.
Upon entering the body, the causative agent concentrates in certain organs and tissues. For example, in the course of evolution the itch mite adapted to parasitizing the epithelial layer of skin; the causative agent of typhus, Rickettsia prowazekii, the wall of arterioles and arterial capillaries; and the influenza virus, the mucous membrane of the upper respiratory tract. After the parasite enters the body, a complex interaction takes place between the parasite and host—an infectious process. The infectious process (dynamics of pathological changes) includes the causative agent’s adaptation to new conditions of existence and its reproduction, dissemination of the process, metastasis, and intoxication of the host. The infectious process and the functional disturbances in the host and its reflex reactions constitute the pathogenetic essence of infectious disease.
An infection is manifested as an acute or chronic form of the disease or carrier state. The development of a particular form depends, on the one hand, on the properties of the causative agent—its infectiousness, invasiveness, and capacity to form exotoxins and endotoxins—and on the number of parasites entering the organism. On the other hand, the condition of the organism and degree of susceptibility or predisposition to a given disease is a very important factor. The presence of certain causative agents in the organism does not provoke a pathological process unless some other conditions exist. Such causative agents are said to be conditionally pathogenic. They include the large group of causative agents of wound infections, many Escherichiaspecies, and herpesvirus. They may exist a long time without producing symptoms on the skin, on the mucous membranes, and in the intestine until an injury, chill, or other factor enables them to manifest their pathogenic properties.
An infected organism or carriers of the causative agents are the sources of infections. Every infectious disease has its own specific mechanism of transmission. In intestinal infections, such as dysentery or typhoid fever, the causative agent is eliminated from the body with feces or urine and in one way or another enters the mouth of a healthy individual. In infectious diseases of the respiratory tract, the causative agent is eliminated with drops of mucus during sneezing, coughing, or talking and penetrates into a healthy individual with inhaled air (the droplet mechanism of transmission). In typhus, malaria, bubonic plague, and some other diseases, the causative agent is trasmitted by blood-sucking insects—lice, mosquitoes, and fleas—parasitizing first a sick and then a healthy individual. The causative agents of scabies and fungal and venereal diseases are transmitted by direct contact with a diseased individual.
A knowledge of the mechanisms of transmission of infection is the basis of prevention of infectious diseases.
REFERENCESMechnikov, I. I. Newspriimchivost’ k infektsionnym bolezniam, 2nd ed. Moscow, 1947.
Gromashevskii, L. V. Obshchaia epidemiologiia, 4th ed. Moscow, 1965. Pages 29–45.
I. I. ELKIN
What does it mean when you dream about an infection?
A dream about being infected might represent anything from absorbing (being “infected by”) the negative attitudes of others to concerns about one’s health. Possibly, the dream infection represents negative thoughts or feelings. (See also Illness).