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In this chapter, we will provide a very brief introduction to the study of taxonomy. Correct diagnosis is essential for treatment and control of any disease and that requires consensus on the names and terms used in the identification process. Without it, there cannot be effective communication between workers both within and between countries. For example, even within a country, a disease or organism may be known by various common names, and language differences further complicate communication. Therefore, before we begin to consider specific parasites, it is necessary to understand of how the taxonomic system works and its relevance to parasitology.

Those who study the identification of organisms are called taxonomists, and they arrange organisms into a hierarchy of categories to demonstrate their relationship to one another. Phylogeny is the study of the evolutionary relationships between organisms. This is increasingly informed by comparisons of gene sequences in a process called molecular phylogeny in which phylogenetic trees are generated to represent the closeness of relationships.

The Ancient Greek philosopher Heraclitus of Ephesus is accredited with the well‐known saying that ‘All is flux. Nothing stays still’. This is certainly true of taxonomy, and frequent name changes and taxonomic re‐arrangements will be a constant refrain throughout this book. One needs to be aware of these changes in order to compare past reports with those published more recently. For example, an organism might now be known under a different name or what was once described as a single species is now considered to consist of two or more distinct species with different biological characteristics.

Over the years, taxonomists have identified numerous organisms and grouped them together in many different arrangements. Primarily, this has been on the basis of their morphology, and this remains a major feature of taxonomy. Increasingly, morphological studies are complemented by molecular phylogeny, and this is having have a major impact on our understanding of animal relationships, confirming some groupings whilst questioning the validity of many others. However, molecular phylogenetics does not always provide clear evidence of the relationships between species. Consequently, there is often a lack of consistency between texts, and there are frequent rearrangements.

There is some debate about how many kingdoms exist although most modern textbooks refer to six: Archaea, Bacteria, Protista, Fungi, Plantae, Animalia. Parasitic species are common in all the kingdoms but traditionally, parasitologists deal almost exclusively with organisms belonging to the kingdoms Protista and Animalia. Although many prokaryotes (archaeans and bacteria) are parasitic, their study falls within a remit of microbiology. Similarly, parasitic fungi fall within the realm of mycology; parasitic plants are reserve of botany (although, these days, many practitioners prefer the title of plant scientist).