Tolerance to infections, or disease tolerance, is one of the mechanisms host organisms can fight against parasites, pathogens or herbivores that attack the host. Tolerance to infections is defined as the ability of a host to limit the impact of parasites

Parasitism

Parasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasite referred to organisms with lifestages that needed more than one host . These are now called macroparasites...

, pathogen

Pathogen

A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...

s or herbivore

Herbivore

Herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...

s on host health, performance, and ultimately on fitness

Fitness (biology)

Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment...

. Tolerance is not equivalent to resistance. Disease resistance

Plant disease resistance

Plant disease resistance is crucial to the reliable production of food, and it provides significant reductions in agricultural use of fuel, land, water and other inputs. There are numerous examples of devastating plant disease impacts , as well as recurrent severe plant disease issues...

 is the host trait that prevents infection or reduces the number of pathogens, parasites and herbivores within or on a host.

Tolerance to infections can be illustrated as the reaction of host performance along with increasing pathogen, parasite or herbivore load. This is a reaction norm

Norms of reaction

In ecology and genetics, a norm of reaction describes the pattern of phenotypic expression of a single genotype across a range of environments. One use of norms of reaction is in describing how different species—especially related species—respond to varying environments...

in which host performance (on y-axis) is regressed against an increasing pathogen, parasite or herbivore burden (on x-axis). 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 number of pathogens, parasites or herbivores. Steep downward slope indicates low tolerance; host performance is strongly reduced with increasing pathogen burden. An upward slope indicates overcompensation; a host can increase its performance with increasing pathogen burden.
In farm animal science, tolerance to infections is sometimes termed disease resilience.