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 life stages that went beyond one host, which are now called macro parasites.

Parasites can now also refer to micro parasites, which are typically smaller, such as viruses and bacteria and can be directly transmitted between hosts of one species.

Not like predators, parasites are usually much smaller than their host, even though both are special cases of consumer-resource interactions. Parasites show a high degree of specialization for their mode of life, and reproduce at a faster rate than their hosts.

The harm and benefit in parasitic interactions concern the biological fitness of the organisms involved. Parasites reduce host fitness in many ways, ranging from general or specialized pathology, impairment of secondary sex characteristics, to the modification of host behavior. Parasites increase their fitness by exploiting hosts for resources necessary for the parasites.

Even though the concept of parasitism applies absolutely too many cases in nature, it is best considered part of a continuum of types of interactions between species, rather than an exclusive category. Particular interactions between species may satisfy some but not all parts of the definition. In many cases, it is difficult to demonstrate that the host is debilitated. In others, there may be no plain occupation on the part of the parasite, or the interaction among the organisms may be short-lived.

Source from: Wikipedia