The degree to which malabsorption occurs in symptomatic and asymptomatic cases is highly varied.The species Giardia intestinalis uses enzymes that break down proteins to attack the villi of the brush border and appears to increase crypt cell proliferation and crypt length of crypt cells existing on the sides of the villi. On an immunological level, activated host T lymphocytes attack endothelial cells that have been injured in order to remove the cell. This occurs after the disruption of proteins that connect brush border endothelial cells to one another. The result is increased intestinal permeability.There appears to be a further increase in programmed enterocyte cell death by Giardia intestinalis, which further damages the intestinal barrier and increases permeability. There is significant upregulation of the programmed cell death cascade by the parasite, and, furthermore, substantial downregulation of the anti-apoptotic protein Bcl-2 and upregulation of the proapoptotic protein Bax. These connections suggest a role of caspase-dependent apoptosis in the pathogenesis of giardiasis.Giardia protects its own growth by reducing the formation of the gas nitric oxide by consuming all local arginine, which is the amino acid necessary to make nitric oxide. Arginine starvation is known to be a cause of programmed cell death, and local removal is a strong apoptotic agent. Host defense
Host defense against Giardia consists of natural barriers, production of nitric oxide, and activation of the innate and adaptive immune systems. Natural barriers
Natural barriers defend against parasite entering the hosts body. Natural barriers consist of mucus layers, bile salt, proteases, and lipases.
duodenalis has been sub-classified into eight genetic assemblages (designated A–H). Genotyping of G. duodenalis isolated from various hosts has shown that assemblages A and B infect the largest range of host species, and appear to be the main and possibly only G. duodenalis assemblages that infect humans. Risk factors
According to the United States Centers for Disease Control and Prevention (CDC), people at greatest risk of infection are:

People in childcare settings
People who are in close contact with someone who has the disease
Travelers within areas that have poor sanitation
People who have contact with feces during sexual activity
Backpackers or campers who drink untreated water from springs, lakes, or rivers
Swimmers who swallow water from swimming pools, hot tubs, interactive fountains, or untreated recreational water from springs, lakes, or rivers
People who get their household water from a shallow well
People with weakened immune systems
People who have contact with infected animals or animal environments contaminated with fecesFactors that increase infection risk for people from developed countries include changing diapers, consuming raw food, owning a dog, and travelling in the developing world. However, 75% of infections in the United Kingdom are acquired in the UK, not through travel elsewhere. In the United States, giardiasis occurs more often in summer, which is believed to be due to a greater amount of time spent on outdoor activities and traveling in the wilderness. Transmission
Giardiasis is transmitted via the fecal-oral route with the ingestion of cysts. Primary routes are personal contact and contaminated water and food.