There are several species of Plasmodium, all causing malaria in birds or mammals. They have complicated life cycles involving two hosts; with sexual stages in a mosquito and asexual stages in mammalian or avian blood.
The Sporozoa are parasitic protozoans that lack locomotor organs. They have no cilia, no flagella, no pseudopods. At some state in their life histories, they are usually intracellular parasites.
They usually pass from host to host in protective capsules called spores which enclose zygotes or juvenile states; however, those species that are transmitted by blood-sucking vectors, like mosquitoes, lack true spores.
The Sporozoa is a very large and diverse class with at least four subclasses and many thousands of species. They cause disease in a wide variety of animals from earthworms and rats to silkworms (the disease called pebrine) and fish.
Malaria is easily the most important sporozoan disease, especially for humans. Coccidiosis, which afflicts poultry and cattle is second. Some sporozoans, like the malarial organism, live primarily in the blood cells; others, like Coccidia, live in the epithelial cells lining the intestine. Still others live in muscles, kidneys, and other organs.

Malaria, after centuries of existence and torment, scientists finally determine its true origin!

Mankind entered a new medical era when the cause of malaria and its mode of transmission were discovered. Descriptions of the disease are found in ancient Chinese and Indian medical writings.
There is good evidence that malaria played an important part in the decline of classical Greece. During the Middle Ages, the rise and fall of human populations in and around Rome appears to have been correlated with the prevalence or absence of malaria.
Even today more people are suffering from malaria than from any other single disease; especially in Africa. Yet, within the lifetime of many men now living, malaria was supposed to be due, as its name implies, to bad air, especially some vague noxious miasma that arose from swamps.
The story of how the cause of malaria was at long last discovered well illustrates the way in which science advances by the cumulative efforts of many individuals. It should be part of the cultural inheritance of every modern man.
A good point of departure is the arrival in London of Ronald Ross, a young army surgeon form India. he went to the famous old St. Bartholomew's Hospital determined to find out all he could about malaria. It was there that he learned the varied facts that had been uncovered and which he later drew together with new ones of his own into a convincing explanation of the true cause of this really great scourge.

1. Ross learned that some fourteen years earlier a French Army physician and parasitologist, Charles Louis Alphonse Laveran (1845-1922), while stationed in Algeria (1878-83), had found microscopic parasites in the blood of malarial patients (1880); and that the parasites were spread through mosquito bites. The experimental demonstration of this was not provided until the late 1890s, by Ronald Ross and other investigators.
• Camillo Golgi (1843-1926), an Italian cytologist, had accurately described how the parasites broke out of the red blood cells and were found free in the blood plasma every time the patient had one of the periodic fever-chills so characteristic of malaria.
• Laveran's discovery in turn had a history that went back to the German pathologist and politician, Rudolf Virchow (1821-1902), and through him to the zoologist and anatomist, Johann Friedrich Meckel (1781-1833).
Meckel discovered that the internal organs of malaria victims were dark, and Virchow then found pigment granules in their cells.
2. Ross also learned of the speculations and observations of Scottish parasitologist, Sir Patrick Manson (1844-1922), and entered into correspondence with him. Manson practiced medicine up and down the China coast.
• He was credited with the discovery that elephantiasis, a disease characterized by great swelling of the legs and caused by minute worm, was transmitted by the bite of a mosquito. He thought that malaria probably was also.
• The work of Manson had its antecedents, too, in the work of Russian naturalist Alexi Pawlowitsch Fedschinko (1844-1873) and through him to German zoologist, Karl Georg Friedrich Rudolf Leuckhardt (1822-1898) and German poet and biologist, Adalbert von Chamisso (1781-1838), who discovered alternation of generations (the recurrence in the life cycle of two or more forms).
3. Several other lines of work contributed to Ross' synthesis. One of these was the development by Russian physician, Dimitri Leonidovitch Romanowsky (1861-1921), of ways to stain blood cells and stain the malarial parasites within them.
• The various blood stains now in common use in every medical laboratory, Wright's stain, Giemsa's stain, and the others, are lineal descendants of Romanowsky's stain.
• Another ingredient that contributed to the final result was the discovery made by Russian naturalist and philosopher, Nikolay Yakovlevich Danilewsky (1822-1865), that many animals; such as, lizards and birds could have malaria, too.

Ross returned to India and, fortified with all this knowledge, was able to make new discoveries that showed conclusively how malaria is transmitted from one person to the next.

1. Ross found that the infection was not carried by bad air but by the bite of a particular kind of mosquito, the Anopheles, or as Ross said, the "dappled-winged mosquito".
2. He discovered the life cycle of the malarial parasite within this species of mosquito. In a way this discovery was the real turning-point, and Ross in his autobiography tells of how late one night, after many fruitless searches, he finally saw the parasites in the gut wall of an Anopheles mosquito.
3. Then in 1897, Ross succeeded in transmitting bird malaria from one bird to another by allowing mosquitoes to bite infected birds and subsequently healthy ones.

Various workers soon demonstrated the transmission of human malaria by mosquitoes. An unpleasant controversy arose as to who really first discovered the cause of malaria.
The report of an international commission resulted in a Nobel prize for Ross; but the controversy illustrates the fact that often when a scientific problem is ready for solution, i.e., when the factual and theoretical background has been built up, several people reach a solution at about the same time.