Intestinal parasites live in your intestinal tract. They are larger than bacteria but usually so small that you cannot see them with the naked eye. You are usually infected by ingesting them after touching something carrying the eggs and then handling food or touching your mouth. Inadequately cooked meat or contaminated water will also contain parasites and their eggs.
There are two major groups of parasites:
1 Protozoans
One-celled organisms like the amoeba which cause Giardia and John Gerard's colitis.
2 Nematodes(roundworms) & Cestodes (tapeworms)
The worms attach themselves to the lining of the small intestine, causing internal bleeding and loss of nutrients. People infested with worms may have no symptoms or may slowly become anemic. Enterobius vermicularis, or pinworm, causes irritation and sleep disturbances.
Nearly all adults, in all parts of the world, carry parasites
Common Parasites
There are a number of common parasites which can live in the human body.
Protozoan
Protozoan are organisms that live in the blood, tissues and intestines. Although exceptionally small, these parasites may remain active in the human body for an entire lifetime, causing multiple complications.
Flukes
Flukes (Trematodes), or flatworms, are likely the most common form of parasite found in humans worldwide. These commonly infect everyone's intestines and other tissues including the heart, lungs, liver and kidneys. All Flukes are terribly difficult to get rid of once infected, accumulating over a period of 10 to 20 years.
Roundworms
Roundworms (Nematodes) include the families of hookworm, pinworm, whipworm, threadworm, etc. Commonly found in the intestines and often contracted through the skin, they, like other parasites, may migrate throughout the body and present multiple symptoms of disease.
Tapeworms
Tapeworms (Cestodes) are generally the largest of the parasites. These segmented worms, containing 50,000 eggs within each of its 3,000-4,000 segments, can release up to one million eggs per day! Some tapeworms may live as long as 25 years and can quickly reach upwards to 10 meters/33 feet in length within the gastrointestinal tract.
However, their larvae can be found in almost any organ, being capable of infecting other tissues in varying stages of its development. Parasites have survival skills which are beyond the capabilities of our Immune System, especially when the immune system is overloaded or weakened, and that's why they can generally live for years within their host.
Some tapeworms which prefer dogs and cats as their hosts can nevertheless be transmitted to humans via petting, grooming and general contact. Children are especially prone to parasites from pets by coming into contain with faeces in gardens, fields and playgrounds. These parasites can then live in a human host. Many tapeworms can live for many years and there have been recorded cases of tapeworms well over 20 years old living inside people.