Good, bad and ugly parasites

Parasitism is a non-mutual symbiotic relationship between species, where one species, the parasite, benefits at the expense of the other, the host.

Unlike predators, parasites do not kill their host. They are also generally much smaller than their host, and will often live in or on their host for an extended period of time. Both are special cases of consumer-resource interactions. Parasites show a high degree of specialization, and reproduce at a faster rate than their hosts. Classic examples of parasitism include interactions between vertebrate hosts and diverse animals such as tapeworms, flukes, the Plasmodium species, and fleas.

However, not all parasites are bad. There are several good parasites that has done more goodwill than harm to mankind. Some examples of good parasites are yeasts, fungis that are used in antibiotics, leeches and maggots that were grown in special laboratories and are used to keep blood flowing or clean up dead tissue after people  had accidents. Therefore, it can clearly be seen that some of these parasites do exist to provide assistance to 

Source :  Claude Combes, The Art of being a Parasite, U. of Chicago Press, 2005