An infection (most modern secondary formation from latin īnficere infect ' , poison'; literally, hineintun ') or infection is the (passive) penetration of pathogens in an organism , where they remain, and subsequently reproduce. Specifically, the pathogens are pathogenic organisms (e.g. bacteria , fungi , parasites ) or molecules (e.g. viruses , transposons and prions ) that need a host to survive . Diseases caused by pathogens are infectious diseases .
The intentional (active) introduction of microorganisms into a nutrient medium is referred to as inoculation and unintentional penetration as contamination . In a figurative sense, the introduction of malware into a computer is also referred to as an infection.
Infections are researched in basic science by infection biology and treated by clinical infectious diseases . The statistical recording of infectious diseases in a population is a sub-area of epidemiology .
The proof of the connection between an infectious agent and an infectious disease has been provided to this day by examining the Henle-Koch postulates . The detection of a pathogen or the immune reaction in a host is carried out through a graduated diagnosis .
Infections occur when pathogens such as bacteria, viruses, fungi, unicellular organisms (see also protozoa ), parasites or prions act on an organism from the outside ( infection pressure ), then penetrate the body, adhere, multiply in it and, except in the case of latent infections, one Trigger a reaction of the body's own defenses ( immune system ). Whether the germs multiply and how severe the infection is depends on the relationship between the germ (“guest”) and the human immune system (host). Most pathogens require a certain number ( minimum infectious dose ) to get into the body to become infected . Frequent side effects of an infection are immune reactions and possibly also a pathogenicity up to lethality , statistically this is expressed in a population by the morbidity and the mortality .
Symptoms of a disease related to an infection are referred to as apparent infectious disease . If an infection does not cause any symptoms, it is called an inapparent or asymptomatic infection. Such infections can nevertheless leave an immune reaction and immunity against further infections with the same pathogen ( silent celebration ).
Colonization and symbiosis
An infection differs from a "settlement" or "settlement" ( colonization ) by commensal bacteria and fungi that live on their skin or mucous membranes without invading the organism. Such a site flora even displaces pathogenic (disease- causing ) germs through competition for space and food and thus forms a very important part of the defense against disease or disease prevention.
If the skin or mucous membrane is damaged or if the immune system is weak , these germs can also cause an infection (endogenous infection). In this context, Staphylococcus aureus is particularly important, which very often causes minor inflammations , but becomes life-threatening in connection with multi-resistance and weakened patients.
Animals need microorganisms to digest their food in the intestines (and the fore-stomachs in ruminants ), just as some of them need their hosts to feed and reproduce (see symbiosis ). Usually this symbiosis remains in balance . But there are germs that break out of this equilibrium and then become dangerous, which is known as opportunistic infection. There are also commensals which, unlike symbionts, do not provide the host with any benefit, but do not penetrate into a healthy host either.
One observation in the pathogenesis in natural hosts is that pathogens adapted to the host usually do not harm it very much, since they need it for their own development and the immune system is activated by cell damage and apoptosis . Avoiding an immune reaction facilitates replication and transmission (synonymous with transmission ) to other hosts. For example, herpes simplex viruses reach infection rates (synonymous with contamination ) of over 90% of the German population with less pronounced symptoms. The simian immunodeficiency virus does not produce AIDS in its natural hosts , unlike HIV in humans. In contrast, infections with Ebola virus in humans, but not in their natural hosts, are occasionally self- extinguishing due to their high virulence, before efficient transmission takes place, since the host is severely weakened and soon dies, consequently its range of motion and thus the spread of the virus limited. A severe course of infection with high mortality (see lethality and mortality ) is usually an indication that the causative pathogen has not yet adapted to the organism in question as its reservoir host. However, the transition from pathogens with a high level of replication (and damage caused) to a permanent infection rate ( Infect and persist , avoiding damage) is fluid. In other words, adapted infectious objects tend to persist and a regulated rate of reproduction, while less adapted pathogens tend to lead to premature termination of the chain of infection . Exceptions are e.g. B. H5N1 viruses in birds, Yersinia pestis and human smallpox viruses in humans. However, the adaptation usually takes place on the part of the host, since the pathogens are in competition with their conspecifics and a less reproductive pathogen would perish more quickly. Therefore, a reduction in pathogenicity in pathogens occurs primarily in connection with an increased rate of reproduction.
The adaptation of the host to the pathogen is referred to as host restriction or resistance. Known antiviral and antibacterial mechanisms in humans include e.g. B. the myxovirus resistance factor Mx1 , the PAMP receptors , the dsRNA-activated inhibitor of translation DAI, the MDA5 , the oligoadenylate synthase OAS1 , the Langerin , the tetherin , the APOBEC3 , the TRIM5alpha and the protein kinase R. In addition, the immune response occurs .
Infections can be classified according to various criteria.
Order of occurrence
- Primary infection (initial infection): the first transmission, i.e. the first contact of the organism with a pathogen.
- Secondary infection (second infection): a transmission of pathogens after the first infection, in addition to and with other pathogens. Such an additional infection can pose considerable problems for the immune system and alsocomplicate therapy and medication (selection and use of drugs ). The course of the disease is usually more severe and shows a variety of symptoms.
- Superinfection (suprainfection):
- Superinfection can also pose considerable problems for the immune system. Therapy and medication are more difficult, the course of the disease is usually more severe with a variety of symptoms.
- Double infection : a simultaneous transmission of two different pathogens.
- Coinfection: the simultaneous presence of at least two different pathogens or variants (subtypes) of the same pathogen.
- Reinfection: Re-infection with the same pathogen after an initial infection has healed (primary infection).
Course of infection
- Transient infection: see Hit and Run
- Persistent infection: see Infect and persist
Infection attributes according to the chronological sequence of the symptoms:
- foudroyant , peracute : fast and dangerous, as the subsequent severe, often fatal course of the disease.
- acute : beginning suddenly, violent effects
- subacute : less violent
- chronic : beginning gradually, extending over a longer period
- recurrent : repetitive with the same pathogen
- latent , persistent : over a long period of time with intervening, clinically silent phases
According to aetiological point of view, a distinction is made:
- Bacterial infection
- Viral infection
- Viral infection
- Prion infection
- Protozoal infection
- Worm infection
- Parasitic infection
- Fungal infection
- Algae infection
Origin and transmission of the pathogen
- Endogenous infection (auto-infection): The pathogen comes from the body's own, normally completely harmless flora. If the immune system is weakened, it reaches z. B. via the skin, from the lungs or from the intestine into the bloodstream.
Exogenous infection: The pathogen comes from the environment. Significant routes of infection are:
- Droplet infection (the pathogen adhering to secretion particles is mainly transmitted when coughing, sneezing and speaking)
- Inhalation infection (for example with varicella, lung plague and pulmonary anthrax)
- Smear infection (contact infection, dirt infection) through touching contaminated objects
- Infection through the exchange of body fluids
- Infection via blood-sucking insects
- Wound infection
- Food infection
- Nosocomial infection : The infection takes place in the hospital, in a doctor's office or other medical facility, and the pathogen comes from the spectrum of germs typical for these locations. The typical bacterial pathogens from the doctor's office or hospital area - e.g. B. Pseudomonads - a high resistance to common antibiotics (drugs for the antimicrobial therapy of infections). Nosocomial infections are also iatrogenic infections if the pathogen is transmitted during medical interventions, for example through catheters or intubation .
- Iatrogenic infection : As with nosocomial infection, the pathogen is transmitted in a medical facility during a medical intervention (to the patient or to the staff), but the pathogen can also be other than the typical hospital pathogens. If, for example, a doctor or nursing staff in a hospital or practice is injuredafter an intravenous injection into an HIV patient with the contaminated cannula andbecomesinfected with HIV, this is called an iatrogenic infection, but not a nosocomial infection. If another patient is infected with a contaminated needle through carelessness, this is also called an iatrogenic infection.
- Polymer- associated infection: Pathogens colonize the plastic surfaces of catheters, artificial heart valves or artificial joints.
Entry gate of the pathogen
Enteral infection: The pathogens enter the organism through the intestines. The entire digestive tract (mouth, throat, esophagus, stomach and the entire intestine) is viewed as the inside of a tunnel that is not itself part of the body. The intestine, from which the infectious agents penetrate the actual interior of the body, is considered here as the gateway.
- Faecal-oral infection: pathogens from faeces enter the organism through the mouth, e.g. B. from contaminated drinking water.
Parenteral infection: In the literal sense, it is an infection in which the pathogens “past the intestine”, ie did not enter the organism via the digestive tract. In medical usage, parenteral is synonymous with “directly into the blood”.
- Percutaneous infection: The pathogens enter the organism through the skin.
- Permucous infection: The pathogens enter the organism via the mucous membranes.
- Inhalation infection : The pathogens enter the organism via the respiratory tract.
- Urogenital infection: The pathogens enter the organism via the urinary tract.
- Genital infection: The pathogens enter the organism via the sexual organs.
- Intrauterine infection: The pathogens enter the body of the unborn child during pregnancy.
Spread of infection
- Local infection : The pathogens stay where they first infected the body (entry point). They only cause symptoms at this point, without spreading further in the organism.
- Generalized infection : The pathogens first multiply at the entry point and then reach their actual organs of manifestation (organs of infection) via the blood. These are often the liver, spleen, lymphatic organs, skin or the nervous system. The pathogens can then no longer be detected at the entry point.
- Focal infection (focal infection): a subsequent (secondary) disease that occurs after local transmission of pathogens by bacteria, especially streptococci . The pathogens get from an initial focus, which has arisen from a local infection in the body, with a delay due to septic metastasis or spurt-wise release from this initial focus via the bloodstream to more distant body regions or organs and cause inflammatory or allergic disease processes there.
- Systemic infection: The pathogens spread through the bloodstream over an entire organ system (for example the central nervous system ) or the entire organism.
Conspicuousness of symptoms or resistance of the organism
Silent (asymptomatic, asymptomatic, inapparent) infection: If the immune system is healthy and the pathogen is adapted to humans, the disease does not break out after the pathogen is transmitted (clinically not manifest, symptom-free). The human then serves as the reservoir host for the pathogen . There are no signs of illness, only a silent celebration takes place (immunization without vaccination and without illness).
- Subclinical infection: The defense mechanisms predominate and prevent the disease from breaking out. The pathogen is eliminated through the development of sterile immunity or a short-term increase in resistance. The infection is limited in time .
Persistent infection: the pathogen lives with the host for an unlimited period of time , but reproduction in the organism is limited and there are no signs of disease. There are several possibilities: developing immunity , increasing the pathogen-specific immune defense, forming interferon or stimulating lymphocytes . Negative influences (e.g. stress) or immunosuppression (e.g. with medication after organ transplantation ) can turn the persistent infection into a manifest infection (clinical symptoms).
- Latent infection: There is a balance between the pathogen and the immune system, indefinitely or until one of the two predominates and either the disease breaks out or the pathogen is killed.
- Tolerated infection: The pathogen, which is usually acquired intrauterine (in the uterus ), can multiply and then be excreted throughout life. However, the host does not become ill unless its immune tolerance is lost.
- Occult (masked) infection: A pathogen invasion has taken place, but the pathogens are neither directly nor indirectly detectable. In the case of symptoms of unexplained cause such as pain and fever, such a hidden infection can be suspected. Under certain circumstances, a virus can be transmitted to daughter cells during cell division and its genome persists in the host cell, but otherwise it cannot be transmitted (temporarily or permanently). So z. B. HBV infection in humans with undetectable hepatitis B antigen ( HBsAg ) is referred to as an occult infection.
- Abortive infection: transmission of pathogens with only mild symptoms.
- Manifest (apparent, clinical) infection: Pathogen transmission with a clear outbreak of the infectious disease (clinical symptoms).
- Opportunistic infection: Transmission of pathogens in people who are already ill with immunodeficiency, which would not lead to disease in healthy people with a normal immune system. The pathogens make use of the body's acquired immune system.
Site of infection (geographical)
- Autochthonous infection: a local infection with the respective pathogen
- Allochthonous infection: an infection brought in (imported) from another location.
Direct and indirect infection
- Direct infection: pathogen transmission from person to person without intermediate steps.
- Indirect infection: transmission through various carriers. This includes vectors such as blood-sucking insects as well as water, food and any objects.
Horizontal and vertical infection
- Horizontal infection: transmission of pathogens from the host to another host of the same generation .
Vertical infection : transmission of pathogens from a host to its offspring.
- Prenatal infection: Pathogen transmission before birth in the uterus (intrauterine) via the placenta (transplacental).
- Perinatal infection: transmission of pathogens during childbirth.
- Postnatal infection: transmission of pathogens after birth, e.g. B. through breast milk.
Prepatent and patent infection (in the case of zoonoses)
- Prepatent infection: in the case of a parasite infection, the phase from the ingestion or penetration of infectious parasite stages into the organism to the development of fully-grown, egg-laying parasites. No reproductive products are found in the host's excretions.
- Patent infection: the phase after the invaders develop into full-blown, egg-laying parasites. Their reproductive products now appear in the host's excretions.
Temporal dynamics of infection and illness
In the temporal course of an infection, a distinction is made between the time of infection, which may be followed by a latency period in which the infected person is not yet infected, followed by an infectious period (infection period), i.e. the period of time during which an infected person can infect others. When looking at the disease, the incubation period is the time from infection to the onset of clinical symptoms.
The predominantly hygienic measures of exposure prophylaxis largely prevent the transmission of pathogens that are responsible for infections. Chemoprophylaxis or post-exposure prophylaxis may also be considered.
Some infectious diseases can also be prevented by vaccination (infection prophylaxis through active immunization).
The treatment of infections depends, among other things, on the causative pathogens. The main considerations are the removal of the source of the infection and antimicrobial therapy with antibiotics.
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