Tolerance to infection, or disease tolerance, is a mechanism that host organisms can use to fight

parasites Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted structurally to this way of lif ...

pathogens In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...

that attack the host. Tolerance is not equivalent to resistance.

Disease resistance Disease resistance is the ability to prevent or reduce the presence of diseases in otherwise susceptible hosts. It can arise from genetic or environmental factors, such as incomplete penetrance. Disease tolerance is different as it is the ability o ...

is the host trait that prevents infection or reduces the number of pathogens and parasites within or on a host. Tolerance to infection can be illustrated via comparing host performance versus increasing load. This is a reaction norm in which host performance is regressed against increasing disease burden. The slope of the reaction norm defines the degree of tolerance. High tolerance is indicated as a flat slope, i.e., host performance is not influenced by increasing burden. Steep downward slope indicates low tolerance in which host performance is strongly reduced with increasing burden. An upward slope indicates overcompensation in which a host increases its performance with increasing burden. Genetic variation in tolerance and its correlation with resistance, can be quantified using random regression model. File:DiseaseToleranceConcept_biggest.png, Tolerance to infections concept illustrated as the host performance response to an increasing pathogen load. In

livestock Livestock are the domesticated animals raised in an agricultural setting to provide labor and produce diversified products for consumption such as meat, eggs, milk, fur, leather, and wool. The term is sometimes used to refer solely to animals ...

science, tolerance to infections is sometimes termed disease resilience. A variety of reactions to pathogens are thought to be involved in tolerance, including superior

immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinte ...

regulation and supplying pathogens with sufficient nutrients to blunt attacks on cells.

Human tolerance

Humans experience tolerance. For example, 90% of people infected with

tuberculosis Tuberculosis (TB) is an infectious disease usually caused by '' Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but it can also affect other parts of the body. Most infections show no symptoms, in ...

experience no symptoms. Similarly, many humans tolerate

helminth Parasitic worms, also known as helminths, are large macroparasites; adults can generally be seen with the naked eye. Many are intestinal worms that are soil-transmitted and infect the gastrointestinal tract. Other parasitic worms such as schi ...

infestations.

Research

Much research makes use of the lethal dose 50 protocol. Subjects are given enough pathogen to kill half of them. The remaining half presumably exhibit the desired tolerance. In many cases, the survivors not only survive but are unaffected by the pathogen. Research is complicated by the fact that animal protocols typically involve expecting some of the subjects to die, which is not ethical in humans.

References

{{Reflist Immunology