According to the FDA "Giardiasis is the most frequent cause of non-bacterial infection in North America". About 2 million people in the U.S. get it every year. It also occurs world wide, infecting an estimated 200 million people. This disease is caused by parasitic protozoans such as Giardia lamblia, G. intestinalis, and G. duodenalis.
According to the Center For Disease Control (CDC) human symptoms can include diarrhea, gas or flatulence, malaise, greasy stools that tend to float, stomach cramps, upset stomach or nausea. Only one third to one half of those people infected with the disease exhibit symptoms. However, infected individuals without symptoms can still spread the disease to others. Fortunately the human immune system is normally able to overcome the parasite in a period of time. Also, there are various medications which can help your recovery. Treatments range from a single dose of one type of medicine to a one week dosage period with another. Rarely some individuals develop chronic conditions. These can be very hard to treat,
People can become infected by ingesting contaminated food or water . The guilty party is a tiny cyst which is part of the Giardia life cycle. A cyst that is ingested is activated by stomach acids and digestive enzymes (Sierra Club web site), lodges in the small intestine, colonizes and reproduces, coating the interior of the small intestine and reducing its ability to absorb nutrients. The infection does not spread into the blood stream or to other parts of the gastrointestinal system. Symptoms develop one to two weeks after infection occurs. The disease cycle usually lasts two to six weeks, occasionally longer. After the Giardia have reproduced they produce tiny cysts which are excreted in feces.
Where do we humans get the cysts? Giardia can live in humans; in wild animals such as beaver, raccoons, squirrels, muskrats, coyotes, bears, and deer; in domestic animals such as pet rabbits, horses, sheep, pigs and cattle; and in pets such as dogs, cats and birds. We, or our children, can also get them through fecal-oral transmission, such as in day care centers, or from diaper changing, or anywhere with poor hygienic practices.
The cysts are very small and range in size from 6 to 10 microns. A micron is one millionth of a meter or 0.000,039 inch. Really small. A Sierra Club site says that means a million could fit under a fingernail. Ten to twenty five cysts are enough to make you sick. The cysts are also extremely hardy. "They can survive weeks to months in cold water". Typically they perish in less than one day when frozen, but may survive many weeks and, in rare cases, as much as one year.
They may also occur in city water reservoirs and persist after water treatment, as the Giardia cysts are resistant to conventional water treatment methods such as chlorination and ozonolysis". (Wikipedia). The cyst is also resistant to the effects of moderate heating, cooling and drying. To kill it you have to freeze it, boil it or completely dessicate it.
Hikers and campers who use surface water are at particular danger of exposure. "Most chemical treatment methods, including common point-of-use treatments such as iodine and chlorine dioxide, are considered unreliable in inactivating Girardi cysts". (Wikipedia) Effective methods of purifying possibly contaminated water are to use a filter of 1 micron or smaller pore size or to heat the water to a roiling boil for 1 minute (CDC).
In normal U.S. society, where 4 to 7 percent of the population is infected and can "pass it on", it makes sense to take precautions. Practice good hygiene (thorough hand washing) and try to see that the members of you family do the same, particularly if there are babies, diapers and child care facilities in your life. What about when you empty that cat box? Avoid water that might be contaminated. Carefully and thoroughly wash raw vegetables and fruit. Avoid contact with fecal matter. That may sound easy, but if an infected child (or an adult, for that matter) has a "toilet accident" while in a swimming pool they may deposit millions of cysts in the water. And remember, the chlorine in the water won't kill the cysts and the water filters are not fine enough to remove the cysts. And certainly, if you yourself (or someone in your family) are sick, or have just recovered from a bout of diarrhea, don't swim in the pool yourself. You might be an active "carrier", even though your symptoms have abated, who can unwittingly infect the water. When traveling in some foreign countries the risk is higher. Remember, there are places in the world where "don't drink the water" makes a lot of sense (or using the questionable water for washing raw fruits or vegetables either).
It is estimated that as much as 20 percent of the population of the world is infected with Giardiasis. Do your best to avoid it. And try to be sure you don't pass it on.
Wikipedia, CDC, FDA and Sierra Club web site articles were major sources of information for this article.