Epidemiology
G. intestinalis is found all over the world, and is one of the most common intestinal infections in the United States (13,1)G. intestinalis has high prevalence rates particularly among young children in third world countries, and research has shown high rates of repeated infection (although often asymptomatic) even within the first year of life (1).  The G. intestinalis cysts are highly infectious, and as few as 10 cysts can cause an infection in an individual. (1) Giardia affects all regions of the United States, with increased incidence in the summer months (likely due to recreational swimming exposure).(6) Outbreaks are also particularly focused around daycares and nurseries, infecting children under 5 years old--and their caregivers--the most. (9,1,12) Below are the most common forms of transmission:

Waterborne transmission:

Waterborne transmission is the most common transmission reported, with numerous documented cases of waterborne epidemics in US (12,15). This includes the consumption of contaminated water—due to unpurified or inadequately purified drinking water—as well as exposure in contaminated recreational water, such as pools or lakes.  There have been multiple documented cases of cysts in the municipal water supply here in the US, although such scenarios do not account for the vast majority of infections (9).

 
source: http://www.telluridewatch.com/archive_news/2005/july/071505/cover-aqua.jpg

Contaminated food:

G. intestinalis can also be passed in food, although this is much less common that waterborne tranmission.  There are many ways food can be fecally contaminated. For example, if a food preparer has unclean hands, or a contamination at a food packaging plant.

Fecal-oral transmission:

This is significant method of transmission, confirmed by the high incidence of G. intestinalis outbreaks in day cares and nurseries. (12).  These outbreaks do not show any seasonality, but rather reflect the close contact between young children, who are significantly more likely to pass the parasite fecal-orally. (12)
source: www.coffeedrome.com/juliephotography.html
 
Sexual transmission:

Some studies suggest that homosexual men may be at increased risk for fecal-oral transmission of Giardia by sexual contact. (11,10,12,1)

Areas of Continued Research:
Is Giardia a Zoonosis?

The jury is still out. It is difficult to differentiate between the Giardia that infects many animals, and the Giardia that infects humans.  Giardia clearly infects domestic animals (cats, dogs), farm animals and wild animals—but there has yet to be definitive evidence of an animal-to-human infection (15).   The animal and human giardia cysts are morphologically indistinguishable, but the genetic differences between them are too great to say for sure that the parasite can infect both animals and humans (15).  There are some studies that establish a likely connection between beavers and wilderness backpackers who are very unlikely to contact Giardia for other people—but this hypothesis is not completely confirmed.(5) Further research must be done to confirm whether or not beavers (or other animals) are indeed reservoirs for Giardia.

source: http://www.in.gov/dnr/fishwild/publications/lifeseries/beavers.jpg

Is recurrence due to reinfection or drug resistance?

It is very hard to tell different Giardia strains apart, so it can be hard to follow epidemiological patterns (12).  Also, diagnosis can be inconsistent, given that the presence of cysts in stool samples can be extremely variable.  Hopefully the use of more accurate diagnostics—such as the ELISA testing—will improve this situation.

Incidence Graphs from the CDC (13)
This graph demonstrates that the highest incidence in the US is in children under 5 years old, and their caregivers.
This graph demonstrates that the highest incidence in the US is in the late summer time.
Country Information
G. intestinalis is globally distributed. There are higher incidence rates in places with untreated water and poor sanitation systems, so many developing countries may have a higher burden of disease (15).