Any organism living in or on another living creature (a host) and taking advantage from it, whilst causing the host disadvantage. In contrast to this, certain organisms live on other creatures that have a symbiotic or mutually beneficia, relationship with their host. Parasites are able to obtain nourishment from the host’s blood, ingested food, or tissues. They may only spend part of their life cycle with the host or can remain there permanently. Some cause little symptoms, whilst others can cause severe disease or even death.
Animal parasites of humans include worms, leeches, ticks, lice, flukes, mites and protozoa. Viruses and disease causing bacteria and fungi are also parasites.