Giardiasis, toxoplasmosis, and malaria are all protozoan diseases that infect a vast majority of the human population each year. Giardiasis, one of the leading causes of non-bacterial gastrointestinal infections in America, infects 2.5 million people annually in the U.S. (7) Toxoplasmosis reportedly infects 22.5% of the adult population in America (3), and an estimated 60 million of the U.S. population are hosts for the parasite.(1) Malaria, arguably the most prevalent protozoan disease, causes close to 500 million cases annually, with fifty percent of the world's population in danger of a possible infection. (6) Giardiasis, toxoplasmosis, and malaria all are overwhelmingly present in the human population because of certain aspects in the lifecycles of the protozoans that cause these diseases.
I believe that the encystment phase of giardia is the reason why giardiasis is so widespread within the human population. Giardia has two phases—the trophozoite phase and the cyst phase. (7) While the trophozoite actually attaches to the intestine of the host, causing symptoms, the cyst phase is responsible for transmission. (7) The cyst form is extremely stable and resistant to environmental elements; it survives well in water and can readily be found in lakes, streams, and particularly pools in the summer. (4) The cyst form of giardia is highly infectious, and only ten of these cysts need to be consumed in order to launch the disease. (4) As a result, once the source (usually water) has been contaminated with feces infected with giardia cysts, it is very easy for the disease to spread to others.
Like giardia, toxoplasma has a resistant, cyst form outside of its host called the oocyst. (9) However, the prevalence of toxoplasma in the human population isn't necessarily due to the inertness of the oocyst. I believe the primary reason for toxoplasma's widespread presence is the ubiquity of the primary host: the cat. Nearly 85 million cats are owned in the United States alone, and an estimated 70 million cats are currently strays. (8) With that many cats defecating in backyards, parks, and other public places, it is no wonder that such a large portion of the American population are carriers for toxoplasma.
In my opinion, malaria is such a notoriously widespread disease due to the overwhelming populations of the female Anopheles mosquito, the vector that carries plasmodium. (2) The female Anopheles mosquito is a cosmopolitan species; the only continent where the insect is not found is Antarctica, which makes malaria a genuine health concern for the majority of the rest of the world. (5) Spread is difficult to contain because mosquitoes are extremely active creatures who can adjust to varying environmental conditions. (5) The Anopheles mosquito has also shown signs of growing resistance to insecticides, making the attempt to lessen the number of vectors problematic, (plasmodium has also increased in resistance to anti-malarial drugs.) (5) Along with these complicating factors, identification of the disease can also be difficult because plasmodium hides inside of the red blood cells of the infected human, thus hidden from the immune system. (2) It is only when the red blood cells lyse that the infected person feels any abnormal symptoms, but these symptoms decrease for a time before resurfacing again. (2) Because of this fact, many infected people do not receive early diagnosis and treatment, a step that is critical in decreasing the epidemics of the disease. (5)
In comparison to bacteria, fungi, and viruses, protozoan diseases are able to infect more people because of the ubiquity of their vectors and primary hosts. Any human can become infected with a protozoan disease due to the route of transmission; any child, teenager, or adult can be bitten by a mosquito or come across infected dirt, soil, sand, or water. Also, protozoan cysts are generally very resistant and can live outside of the host for longer periods of time than viruses, who cannot sustain outside of its host cell really at all. Bacteria face far too many threats from outside sources, such as bacteriophages and antibacterial agents, which inhibit spread, while protozoans generally do not. Fungi generally cause harm to the skin, making the disease easily detectable within the victim, and thus allowing treatment of the patient to transpire. As previously stated, this is not so in malaria, as the infected often aren't diagnosed until the late stages of the disease. (10)