HUMANS SHARE planet Earth with countless living organisms, many of which affect human health. As every ophthalmologist will recall from undergraduate courses in biology, the taxonomy of these living creatures classifies related organisms into categories. In descending order, these are as follows: kingdom, phylum, class, subclass, superorder, order, suborder, superfamily, family, genus, and species. The roundworms are included in the phylum Nematoda, which is the largest and most widespread group of multicellular organisms. The evolution of numerous nematodes has resulted in complex life cycles in which the worm is parasitic to invertebrates and vertebrates during part of its cycle. Often the organism is spread from one host to another through the bite of an arthropod vector or the eating of meat containing a larval stage of the worm. Sometimes the life cycle requires specific species as vectors and intermediate hosts. Some human infections due to nematodes are primarily zootic disorders in which the species Homo sapiens has become an accidental intermediate host.
Gordon K. Klintworth. A Worm Can Be Monkey Business. Arch Ophthalmol. 2002;120(5):634–635. doi:10.1001/archopht.120.5.634