Traditionally parasitology has been concerned with the harmful effects of organisms living and feeding inside or on another living organism. Although parasites may belong to any of the following groups – bacteria, yeast, fungi, algae, viruses, protozoa, helminths and arthropods – in effect parasitologists have focused on the internal zooparasites. Bacteria, viruses and fungi of plant origin are the provenance of microbiologists. Before the discovery of unicellular parasites c. 1880s onwards, worms or helminths were the major focus of attention.