Humans are afflicted by a number of diseases caused by parasitic protozoa and helminth worms. The first records of these ancient associations come from studies on archeologic material and the writings of the Greek, Egyptian, and Roman empires, but it was not until the theory of spontaneous generation had been disproved in the nineteenth century that it became possible to incriminate parasites in the etiologies of a number of diseases that had long plagued mankind.
The golden age of parasitology was the nineteenth century, when most of the life cycles of parasites were accurately described for the first time.
As an overview, here are a few of the most commonly written-about parasites in history. Keep in mind, the microscopic parasites (mostly the protozoa) were not known, in that they were not seen in those that perished from the subsequent diseases, but they were known for the diseases they caused; malaria, amoebic dysentery, and giardia are all protozoan parasitic diseases that have killed millions throughout history.
Ticks, fleas, lice, and scabies are all ectoparasites, meaning that they live outside the body or just within the skin. They can be incredibly dangerous, thanks to the bacteria and viruses they can transmit. I’ll cover those in a bit! For now, here are some most common endoparasites.
Endoparasites - Parasites that cause infection inside the body.
Plasmodium spp.: Protozoan - Eleven species that infect humans (four significant species), causes malaria, transmitted by Anopheles spp. mosquitoes.
Schistosoma spp: Flatworm - Causes schistosomiasis (also known as bilharzia), which, while rarely fatal, can cause chronic illness and organ damage, cause genital sores (increasing susceptibility to HIV), as well as abdominal pain and diarrhea. There are descriptions of schistosomiasis-like diseases as long ago as the early Greek/late Egyptian writings.
From Invertebrate Anatomy. R. D. Barnes, 1980.
Strongyloides spp: Roundworm - Commonly known as “hookworm”. Was the chief cause of the “lazy Southerner” phenomenon. Infection can cause anemia, dysentery, and hemorrhage. Infection in children is particularly problematic, as it causes drastically increased absences from school, learning disabilities, and slowed motor function development - this sort of childhood leads to unproductive adults.
Ascaris lumbricoides: Roundworm - Known as the “giant intestinal roundworm”. Lives in the small intestine and feeds upon chyme. Ascariasis is the most common helminth infection of humans, and while often asymptomatic for years, it can cause extreme symptoms and even death when symptoms manifest.
Guinea Worm: Roundworm - Caused by Dracunculus medinensis and causes “Guinea Worm disease” (GWD) (also known as dracunuliasis - from a Latin term meaning “affliction with little dragons”). Lives within the bloodstream, and is transmitted through contaminated water.
Tapeworm: Flatworm - We all known tapeworms, right? There are several species of Cestoda that infect humans, mostly coming from under-cooked meat products. Generally inhabits the intestine, but can sometimes (especially pork tapeworms - Taenia solium) encyst within muscle tissues, and eggs can even pass blood-brain barrier. If the body doesn’t destroy the small larvae within the brain, they encyst and cause horrible swelling and destruction of areas of the brain. Don’t worry, that kind of infection is incredibly rare in general, but is practically uneheard when you fully cook or don’t eat pork (and don’t have a maid/cook who already has tapeworms - interesting story, will go into it later)
From commons.wikimedia.org, the Wikimedia Commons Archive
Entamoeba: Protozoan - Causes amoebic dysentery and amoebic liver cysts, and is transmitted by contaminated water. Caused at least several hundred deaths along the “Oregon Trail” (well, it was along all the migration trails, even the drier trails to California). Entamoeba histolytica is the species that causes amoebiasis. Don’t confuse Entamoeba coli with Escherischia coli! The former is a protozoan, while the latter is what common culture knows as “E. coli”.
United States Center for Disease Control Graphic - E. histolytica life cycle
Giardia: Protozoan - The giardia are creepy little guys. They’re flagellated protozoans that swim/stick themselves down in the intestines and cause giardiasis. One becomes infected with giardia by ingestion of fecal material or contaminated soil. Though they can cause extreme abdominal cramps, diarrhea, and gas, giardiasis does clear up on its own after 2-6 weeks. What one has to avoid is re-infection…since giardia can be free-swimming, they can survive within water sources on their own, without a problem. That’s why outbreaks in small communities without the ability to sanitize water do not cease without education about boiling water, or intervention leading to sanitized water comes around.
Giardia populating a gerbil intestine: National Institute of Health (NIH) scanning electromicrograph image