Medical parasitology traditionally has included the study of three major groups of animals: parasitic protozoa, parasitic helminths (worms), and those arthropods that directly cause disease or act as vectors of various pathogens. A parasite is a pathogen that simultaneously injures and derives sustenance from its host. Some organisms called parasites are actually commensals, in that they neither benefit nor harm their host (for example, Entamoeba coli). Although parasitology had its origins in the zoologic sciences, it is today an interdisciplinary field, greatly influenced by microbiology, immunology, biochemistry, and other life sciences. Infections of humans caused by parasites number in the billions and range from relatively innocuous to fatal. The diseases caused by these parasites constitute major human health problems throughout the world. (For example, approximately 30 percent of the world's population is infected with the nematode Ascaris lumbricoides.) The incidence of many parasitic diseases (e.g., schistosomiasis, malaria) have increased rather than decreased in recent years. Other parasitic illnesses have increased in importance as a result of the AIDS epidemic (e.g., cryptosporidiosis, Pneumocystis carinii pneumonia, and strongyloidiasis). The migration of parasite-infected people, including refugees, from areas with high prevalence rates of parasitic infection also has added to the health problems of certain countries. A misconception about parasitic infections is that they occur only in tropical areas. Although most parasitic infections are more prevalent in the tropics, many people in temperate and subtropical areas also become infected, and visitors to tropical countries may return with a parasite infection. The unicellular parasites (protozoa) and multicellular parasites (helminths, arthropods) are antigenically and biochemically complex, as are their life histories and the pathogenesis of the diseases they cause. During their life, parasitic organisms typically go through several developmental stages that involve changes not only in structure but also in biochemical and antigenic composition. Some helminth larval stages have little resemblance to the adult stages (for example, those of tapeworms and flukes). Some parasitic protozoa also change greatly during their life history; for example, Toxoplasma gondii is an intestinal coccidian in cats but in humans takes on a different form and localizes in deep tissues. Some of these infections can convert from a well-tolerated or asymptomatic condition to life-threatening disease. Many parasitic infections are transmitted from animals to humans (zoonotic infections); the human disease may or may not resemble the disease caused in the lower animal host. The information can be published in our peer reviewed journal with impact factors and are calculated using citations not only from research articles but also review articles (which tend to receive more citations), editorials, letters, meeting abstracts, short communications, and case reports. The inclusion of these publications provides the opportunity for editors and publishers to manipulate the ratio used to calculate the impact factor and try to increase their number rapidly. Impact factor plays a major role for the particular journal. Journal with higher impact factor is considered to be more important than other ones.