Plasmodium vivax
Plasmodium vivax is a protozoal parasite and a human pathogen. This parasite is the most frequent and widely distributed cause of recurring (Benign tertian) malaria, "P. vivax" is one of the five species of malaria parasites that commonly infect humans. Although it is less virulent than "Plasmodium falciparum", the deadliest of the five human malaria parasites, "P. vivax" malaria infections can lead to severe disease and death, often due to splenomegaly (a pathologically enlarged spleen). "P. vivax" is carried by the female "Anopheles" mosquito; the males do not bite.
Yersinia pestis
Yersinia pestis (formerly "Pasteurella pestis") is a gram-negative, nonmotile, rod-shaped coccobacillus, with no spores. It is a facultative anaerobic organism that can infect humans via the Oriental rat flea. It causes the disease plague, which takes three main forms: pneumonic, septicemic and bubonic plagues. All three forms were responsible for a number of high-mortality epidemics throughout human history, including: the sixth century's Plague of Justinian; the Black Death, which accounted for the death of at least one-third of the European population between 1347 and 1353; and the Third Pandemic, sometimes referred to as the Modern Plague, which began in the late nineteenth century in China and spread by rats on steamboats claiming close to 10,000,000 lives. These plagues likely originated in China and were transmitted west via trade routes. Recent research indicates that the pathogen may have been the cause of what is described as the Neolithic Decline, when European populations declined significantly. This would push the date to much earlier and might be indicative of an origin in Europe rather than Eurasia.