ABSTRACT
The term protozoa is derived from 'proto' meaning 'first' and 'zoa' meaning 'animal'. Protozoa are 'first animals' which generally describes their animal like nutrition. Protozoa are the largest group of single-celled, microscopic organisms with more than 20,000 species that are found in all aspects of life. Protozoa are widely distributed in various environments from favourable rainforests to sandy beaches to the bottom of oceans to snowcovered mountains. With the availability of improved diagnostic methods, protozoa are being discovered from diverse habitats. However, the abundance and diversity of protozoa in ecosystems is dependent on abiotic factors such as water, temperature, pH, salinity, osmolarity and biotic factors including the availability of food particles. Protozoa include the causative agents of some of the most notorious and deadly diseases. For example, malaria alone causes between one to two million deaths worldwide, annually. Other protozoa play important roles in the food chain maintaining a balanced ecosystem or act as commensal organisms (not harmful) of nearly all humans. Some of the protozoan pathogens have only recently been identified as a major threat to human health. For example, Cryptosporidium was originally described in the 19th century, but has recently been associated with serious human infections in AIDS patients. With the increasing number of AIDS patients during the last few decades, many of the protozoan pathogens have become a major problem to human health.
