Infection

Infection (probably modern secondary formation from Latin īnficere 'to infect', 'to poison'; literally 'to put into') or contagion is the (passive) penetration of pathogens into an organism, where they remain and subsequently multiply; in plants, this is also referred to as infestation. The purely mechanical process by which infectious agents come into contact with the host is called infection. If the infectious agent does not settle in the host after contact, there is no adhesion of the agent in the macroorganism and the infection event is terminated. Specifically, pathogens are pathogenic organisms (e.g. bacteria, fungi, and parasites) or molecules (e.g. viruses, transposons, and prions) that require a host to survive. Diseases caused by infection with pathogens are called infectious diseases.

The unintentional introduction of microorganisms, viruses, viroids and prions into a nutrient medium is referred to as contamination, the intentional (active) introduction as inoculation and the presence and growth without virulence as colonisation (see also colonisation (medicine); example intestinal bacteria). In a figurative sense, the introduction of malware into a computer is also referred to as infection.

Infections are researched in basic science by infection biology and treated by clinical infectiology. The statistical recording of infectious diseases in a population is a subarea of epidemiology.

The proof of the connection between an infectious agent and an infectious disease is still provided today by reviewing the Henle-Koch postulates. The detection of a pathogen or the immune response in a host is carried out by means of a graded diagnostic procedure.

Pathophysiology

Infections occur when pathogens such as bacteria, viruses, fungi, protozoa, parasites (for example in worm diseases) or prions act on an organism from the outside (infection pressure), then penetrate the body, adhere to it, multiply in it and (except in the case of latent infections) trigger a reaction of the body's own defences (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 to enter the body for an infection to occur (minimum infectious dose). Common concomitants of infection are immune responses and possibly pathogenicity to lethality. Morbidity and mortality in a population are statistical measures of this.

Symptoms of a disease associated with an infection are called apparent infectious disease. If an infection does not cause any symptoms, it is called an inapparent or also asymptomatic infection. Such infections can nevertheless leave an immune response and immunity against further infections with the same pathogen (silent infection).

Colonization and symbiosis

An infection differs from a "settlement" or "colonization" (colonization) by commensal bacteria and fungi, which live on their skin or mucous membranes without penetrating the organism. Such a local flora even displaces pathogenic (disease-causing) germs through competition for space and food and thus forms a very important part of the defence against or prevention of disease.

In the case of damaged skin or mucous membrane or immunodeficiency, however, these germs can also cause an infection (endogenous infection). In this context, Staphylococcus aureus is particularly significant, 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 gut (and the forestomachs in ruminants), just as some of these need their host to feed and reproduce (see symbiosis). Most of the time, this symbiosis remains in equilibrium. However, there are germs that break out of this equilibrium and then become dangerous, which is called an opportunistic infection. In addition, there are commensals which, unlike symbionts, do not benefit the host, but also do not invade a healthy host.