Parasitism, a symbotic relationship, involves a species of organism (parasite) that infects a host species and depend on the host for nutrition and other basic metabolic needs. In highly evolved types of parasitism, minimal damage may be done to a host population, except that less-fit or disabled members may be weeded out. At the other end of the spectrum, highly virulent parasites cause diseases, such as rabies, and are referred to as pathogens.
Almost every life-form is affected by parasitism. Even bacteria are infected by certain viruses known as bacteripages. Many parasites cause considerable agricultural damage. Tapeworms, which live in gastroinestinal tract, are among the parasites that cause human illness.
Some parasitic diseases more detail describe on http://bacteria.health-tips-diseases.com
