An extraordinary variety of viruses, bacteria, and parasites stand ready to attack us and feed off our bodies' cells. Using microscopes of varying power, photographers show us a teeming microbial world that we could barely imagine without their help.

The approximate sizes of microbes can be approximated by using the following rule of thumb:

• VIRUSES are the smallest of all infectious agents, averaging about 100 nanometers (100 billionths of a meter) in length. They have so few genes and proteins of their own that in order to reproduce they need to commandeer the machinery of the cells they invade.



• BACTERIA vary widely in size and shape, but tend to be at least 10 times larger than viruses, or at least 1 micrometer (1 millionth of a meter) long. They are single-cell organisms that reproduce independently.



• SINGLE-CELL PARASITES tend to be at least 10 times larger than bacteria, or about .01 millimeter long.



• MULTI CELLULAR PARASITES are so large they can usually be seen with the naked eye. Tapeworms, for instance, can reach a length of 6 meters (20 feet).

Food and water are the most common sources of parasite transmission. Since most of us eat three times a day and drink water frequently throughout the day, our exposure to these sources is constant. Tap water has been found to be contaminated with parasitic organisms. Both plant and animal foods carry parasites, and cleaning and cooking methods often do not destroy them before ingestion. The CDC (Center for Disease Control) cites food as the catalyst behind 80 percent of the pathogenic outbreaks in the U.S. Most are linked to restaurants and delis where less than sanitary conditions exist -- from food preparation and storage to the utensils and servers' hands.

Animals, just like humans, can become infected with parasites. Internally, contaminated water and food can spread the problem to our pets. Externally, animals become infected by parasites on their bodies, especially on their fur, because of exposure to infected animal wastes. Forgetting to wash your hands even one time after handling or cleaning up after your animal can transmit the parasite to you. Pets are a wonderful part of our lives. They provide comfort, companionship, protection, amusement, and unconditional love for their owners. Yet, pets, like humans, are often victims of serious infections that can unintentionally be passed on to their owners. In fact, there is a whole set of diseases classified as 'zoonoses' (animal-transmitted diseases) in parasitology textbooks. Animals are major carriers of parasites, and most physicians, let alone the general public, are unaware of this fact. Experts have projected that of the 110 million pet dogs and cats in this country, over half may be infected with at least one or more different kinds of parasites. Considering these numbers, the potential for transmission of parasitic infection from animals to humans is extremely high.

The CDC estimates that the number of parasites present in the United States alone number in the thousands. These parasites are biochemically complex creatures in their life histories, development, reproductive cycles, nutritional requirements, and disease manifestation. They are categorized according to structure, shape, function, and reproductive ability. These include microscopic organisms (protozoa); roundworms, pinworms, and hookworms (nematoda); tapeworms (cestoda); and flukes (trematoda).

PROTOZOA
Making up approximately 70 percent of all parasites, protozoa are invisible to the naked eye. They are one-celled microscopic organisms, but don't let their size fool you. Certain protoans, through their intensely rapid reproductive ability, can take over the intestinal tract of their host; and from there go on to other organs and tissues. Some feed on red blood cells. Some protozoa produce cysts - closed sacs in which they may be safely transported through food and water from one person to another. In the cyst state, protozoans are safe from destruction by human digestive juices. These one-celled 'vampires' can actually destroy the tissues of their hosts. According to experts, an estimated 7 million people across the U.S. have some form of protozoa living inside of them. Common protozoa include: Endolimax nana, Giardia lamblia, Entamoeba histolytica, Cryptosporidum parvum, Blastocystis hominis, Trichomonas vaginalis, Toxoplasma gondii, Cyclospora cayetanensis, Cryptosporidium muris, Pneumocystis carinii, Plasmodium malariae, Plasmodium ovale, Plasmodium vivax, Plasmodium falciparum, Leishmania donovani, Leishmania tropica, and Leishmania braziliensis.

NEMATODE
While the protozoans are only single-celled, nematode creatures are multi cellular. The adult worms multiply by producing eggs called ova or larvae. The eggs usually become infectious in soil or in an intermediate host before humans are infected. It is interesting to note that unless the worm infection is heavy, many individuals do not show signs of disease. While it may be unpleasant to consider, it is true that the human host can coexist quite comfortably with a few worms, unless they reproduce in great numbers and create organ obstruction. Experts claim that 'some type of worm is already in the intestines of over 75 percent of the world's population'. This is a frightening statement. Common nematode include: Roundworm (Ascaris lumbricoides), Hookworm (Necator Americanus, Ancylostoma duodenal), Pinworm (Enterobius vermicularis), Roundworm (Toxocara canis, Toxocara cati), Heart worm (Dirofilaria immitis), Strongyloides (Stronglyoides stercoralis), Trichinella (Trichinella spiralis), Filaria (Wuchereria bancrofti, Brugia malayi, Onchocerca volvulus, Loa loa, Mansonella streptocerca, Mansonella perstans, Mansonella ozzardi), and Anisakine larvae.

CESTODA
Among the oldest known parasites, tapeworms are considered humanity's largest intestinal inhabitant. They each have a scolex (head) that attaches to the intestinal wall. As long as the head remains attached to the intestinal mucosa, a new worm can grow from it. Tapeworms do not contain digestive tracts but get their nourishment by absorbing partially digested substances from the host. They are whitish in color, flat, and ribbon-like, with a covering that is a transparent skin-like layer. Common cestoda include: Beef tapeworm (Taenia saginata), Pork tapeworm (Taenia solium), Fish tapeworm (Diphyllobothrium latum), and Dog tapeworm (Dipylidium caninum).

TREMATODE
Trematode are leaf-shaped flatworms also known as flukes. They are parasitic during nearly all of their life-cycle forms. The cycle begins when larvae are released into freshwater by infected snails. The free-swimming larvae can then directly penetrate the skin of the human host or are ingested after encysting in or on various edible, vegetation, fish, or crustaceans. Common trematode include: Intestinal fluke (Fasciolopsis buski), Blood fluke (Schistosoma japonicum, Schistosoma mansoni) Schistosoma haematobium), Liver fluke (Clonorchis sinensis), Oriental lung fluke (Paragonimus westermani), and Sheep liver fluke (Fasciola hepatica).

Roundworm Click here for more info.	Pinworm Click here for more info.
Hookworm Click here for more info.	Dwarf Tapeworm Click here for more info.
Whipworm Click here for more info.	Fish Tapeworm Click here for more info.
Intestinal Fluke Click here for more info.	Amoeba Parasite Click here for more info.

Click here for Parasite Articles
Note: The statements contained on this website have not been reviewed or approved by the Food and Drug Administration for their validity. Nothing contained on this site is meant to infer or state that the products are for the treatment of any disease or ailment. Always consult with your physician if you experience any medical problems.

Source: Parasite images courtesy of the DPD

---

References and additional information:
- Centers for Disease Control & Prevention
- Howard Hughes Medical Institute
- Ohio State University, Biological Sciences
- Skye Weintraub, ND "The Parasite Menace""; Woodland Publishing 2000
- Ann Louise Gittleman, MS, CNS "Guess What Came To Dinner?"; Avery 2001
- Valerie Saxion "Everybody Has Parasites"; Bronze Bow Publishing 2003
- Skye Weintraub "The Parasite Menace"; March 1998
- Roger M. Knutson "Fearsome Fauna: A Field Guide to the Creatures That Live in You"
- Carl Zimmer "Parasite Rex: Inside the Bizarre World of Nature's Most Dangerous Creatures"
- Paavo Airola, ND, PhD "How To Get Well"; Health Plus Publishers
- Nicholas Culpepper "Culpepper's Complete Herbal"; Omega 1985
- Penny C. Royal "Herbally Yours"; Sound Nutrition 1982
- James F. Balch, MD "Prescription For Nutritional Healing"; Sound Nutrition 1997
- Alma R. Hutchens "Indian Herbology of North America"; Merco 1973
- Discover Magazine; August 2000 Edition