Abstract This study is an analysis of Leishmaniasis Disease New World as a protozoan disease. To provide a critical analysis of the disease, this study provides a differential diagnosis and an epidemiology of the disease, to provide a clear understanding of the symptoms of the disease and its transmission. In the diagnostic testing process, this study notes that, special expertise needs to be included as part of the diagnosis testing process, and in comparison to other diagnostic methods, the process is expensive. This is one drawback of the diagnosis process. However, this study outlines the standards of care to be undertaken in the diagnosis process and establishes that the disease can be treated through drug administration. Differential Diagnosis Leishmaniasis is a disease spread through protozoan infection, when a patient is bitten by a sand fly. New world leishmaniasis is realized when a patient is bitten by new world sand flies as a special strain of the disease (Badaro, 2006). Once a patient is infected, he or she may show different strains of severity of the disease, but the most severe form of Leishmaniasis is the visceral leishmaniasis, and the most common strain is the cutameous leishmaniasis (Badaro, 2006). Leishmaniasis presents different forms of diagnosis. Leishmaniasis is normally diagnosed in the haematology lab, where amastigotes are observed through careful examination to determine a patient’s status (with regards to the disease). Other diagnostic elements of the disease involve the observation of skin rashes, or in extreme cases, skin cancer. In extreme cases (when the patient has been severely affected), the patient may exhibit signs of vital organ complications, with symptoms such as lung dysfunction (to resemble tuberculosis and liver dysfunction) or liver dysfunction being prominent (Badaro, 2006). In order of priority, the observation of amastigotes in the laboratory sample is the first course of diagnosis. Skin rashes or skin infection, is the second factor in the line of diagnosis, while vital organ dysfunction is the last course of diagnosis. To support this diagnosis criterion, we should critically look at the stages of leishmaniasis development. When a patient is affected with leishmaniasis protozoa, the first sign that, the patient is infected is normally exhibited in the patient’s tissues because the amastigotes are directly visible in the patient’s tissues upon infection (Trudel, 2008, p. 1292). Skin rashes may occur almost at the same time, but the reason for prioritizing laboratory check as the first diagnostic element is the accuracy of the laboratory checks performed. Skin rashes can be caused by several factors and this is why it is placed second in the diagnostic list. Lastly, during long periods of infection, leishmaniasis disease easily spreads to affect the functions of the vital organs, but this diagnosis takes very long to detect. This is the reason this line of diagnosis is the third. Epidemiology of the Diagnosis The epidemiology of the diagnostic processes described above stems from the fact that leishmaniasis is pathogenic and it is transmitted by the insect new world sand fly. Since the insect greatly depends on the protein compounds found in human blood, for its reprod