An organism that lives in or on another organism, called the host, and obtains resources from it, often causing harm.
Parasites depend on their hosts for some aspect of survival, growth, or reproduction. Human parasites include microscopic organisms such as protozoa and larger organisms such as parasitic worms (helminths).
Not every interaction between organisms in which one benefits and another is affected is classified as parasitism; the term specifically refers to a biological relationship in which the parasite benefits while the host is generally harmed.
QUICK FACT
Not all parasites are worms. Malaria, for example, is caused by Plasmodium, a microscopic protozoan parasite.
COMMONLY CONFUSED WITH
Infectious agent: a broader term for any biological agent capable of causing infection; parasites are one type of infectious agent.
SEE ALSO
