Of the three common species of malaria plasmodia, P. vivax, P. malariae and P. falciparum, the latter is far the most pathogenic. This greater pathogenicity is observed and reported wherever malaria prevails throughout the world. The number of deaths caused by infection with P. vivax and P. malariae is insignificant as compared with the number produced by P. falciparum. In fact, practically all deaths caused by malaria are caused by P. falciparum and almost none by the other species. Not only are nearly all deaths caused by it, but it causes almost all of the pernicious clinical types of malaria.
One of the explanations offered for this greater pathogenicity is that the organism produces a more harmful or more powerful toxin. This explanation is theoretical entirely. Another explanation offered is the larger number of parasites that are present in the blood of many of the more pernicious and destructive cases.