Parasitism is a type of symbiotic
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...

 relationship between organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

s of different species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 where one organism, the parasite, benefits at the expense of the other, the host
Host (biology)
In biology, a host is an organism that harbors a parasite, or a mutual or commensal symbiont, typically providing nourishment and shelter. In botany, a host plant is one that supplies food resources and substrate for certain insects or other fauna...

. Traditionally parasite referred to organisms with lifestages that needed more than one host (e.g. Taenia solium). These are now called macroparasite
Macroparasites are parasites that are large enough to be seen with the naked eye, in contrast to microparasites. They grow in one host but reproduce by infective stages outside of this host. These generally include ticks, mites, nematodes, flatworms, etc., and can be either external parasites or...

s (typically protozoa
Protozoan infection
Protozoan infections are parasitic diseases organisms formerly classified in the Kingdom Protozoa. They include organisms classified in Amoebozoa, Excavata, and Chromalveolata....

 and helminths). The word parasite now also refers to microparasite
A microparasite is a parasite that complete a full life cycle within one host and can be transmitted directly to conspecific hosts. They often reproduce within a host's cells and are generally too small to be seen with the naked eye. Most are viruses, bacteria and fungi with a smaller number being...

s, which are typically smaller, such as viruses and bacteria, and can be directly transmitted between hosts of the same species.

Unlike predators
In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...

, parasites are generally much smaller than their host; both are special cases of consumer-resource interactions
Consumer-resource systems
Consumer-resource interactions are the core motif of ecological food chains or food webs, and are an umbrella term for a variety of more specialized types of biological species interactions including prey-predator , host-parasite , plant-herbivore and victim-exploiter systems...