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.
Karl Friedrich Meyer
Meyer also investigated what are called arbovirus diseases, among them equine encephalitis. Several diseases transmitted from animals to humans are due to a group of viruses carried by mosquitoes. Mosquitoes belong to the ‘animal family’ (technically a ‘phylum’) called arthropods; hence the viruses carried by them are "ar"thropod-"bo"rne. Different arboviruses may cause diverse diseases. Here, we summarize examples of Meyer’s outstanding discoveries in this area. 
In the early 1930s, thousands of horses in California suffered and died from a paralytic disease, later called western equine encephalitis. Meyer proved that it was of viral origin. Later it became clear that the virus can also infect humans causing encephalitis, sometimes with deadly complications. – Meyer and his colleagues at the Hooper later demonstrated that mosquitoes pick up the virus from chicken and (migratory) birds and transfer it to man and to horses. It also became clear that the disease is common near irrigated fields where mosquitoes abound. Meyer therefore qualified encephalitis as a man-made disease. Subsequently, a vaccine for horses was developed. And it turned out that the isolation of the virus by Meyer led to the discovery of similar kinds causing other types of encephalitis in man. (Other microbiologists found related types of viruses in other animals than horses.) – Effective mosquito control was the key to minimizing this problem.