The main categories of parasites that affect humans are protozoa and so-called "worms." Both are endoparasites, parasitic beings that live inside the host to their benefit and the host's harm. Ectoparasites live on the outside of the host (like ticks), and will not be addressed in this article.
Protozoa are single-celled organisms similar to animals, though usually categorized in their own biological kingdom, Protozoa. There are many protozoa in the world, found mostly in water and soil. They are important agents in the ecology of soil, aiding the break down of biomass and also controlling the population of bacteria, as bacteria are one of their main food sources. However, some types of protozoa are harmful to humans when they enter the human body.
The most common protozoan parasite (and most common parasite) is Giardia. This is most commonly contracted by drinking unpurified water from a river, stream, or lake. However, Giardia is also one of the least harmful parasites, though it will cause great discomfort while it runs it course and pharmaceutical medicine is recommended to speed and aid recovery. Other parasitic protozoa that affect humans include trypanosome and leishmania.
The more harmful protozoa are those that form protective cysts around themselves, making them difficult to kill with remedies (herbal or pharmaceutical) because of the hard cyst. The cyst also allows the protozoa to survive outside the host, making it able to survive until another host picks it up.
Worms are in the animal kingdom, though they are unrelated to the worms you find in your garden soil. Intestinal "worms" can be one of several categories, including trematoda, cestoda, and nematoda.
Nematodes, also known as round worms, live in both aqueous and terrestrial environments. Nematodes shed their outer layer (akin to an exoskeleton), as they grow larger. Most are microscopic.
Trematodes, also known as flukes, and are almost all parasites. They primarily live in aqueous environments and parasitize marine life, like shell fish and fish. Flukes infest either body tissue (lungs, organs, or muscles) or the blood. They are contracted through water, feces, or undercooked food.
Cestodes, also known as flatworms, are the ones featured in horror films and biology books. These are the suckers that get BIG. The most common is the tapeworm, which can grow many feet long when untreated. The tapeworm attaches itself inside the host, almost always in the intestines. There are pharmaceutical medications for tapeworms that your doctor can recommend if you encounter one of these suckers.
Viruses are a weird phenomenon, not exactly living, but not exactly non-living either. They use the host to multiply themselves, by hijacking the DNA and cellular reproduction of the host to reproduce the viruses own genetic material. .
Most parasites are detected through the patient's feces. IT is recommended to take fecal samples on multiple days if you suspect you have parasites. Your doctor can usually provide testing or recommend a lab to provide testing, but be sure to request multiples days worth of samples as that is not the norm, but can make a big difference because some parasitic lifecycles take multiple days and therefore may not be detected by a one day sample. Even the tapeworms, while can get big, they are usually identified through eggs in the patients feces, so this test is essential to diagnosis if you suspect parasites. If you are really worried you can also request blood work, though blood parasites are less common.
While parasites abound in the earth, the human body is generally able to deal with encounters successfully. Your stomach acid will kill most of them if they enter through water or raw food or improper food handling. Individuals with weakened immune systems are at greater risk and should proceed with caution when rough backpacking and choosing restaurants.