Giardia is an intestinal parasite that. like all others, uses the host for infection. Giardia also infects dogs, cats, and beavers as well as humans and this is how water gets contaminated and infects humans.
People usually become infected either hand to mouth from unwashed hands or by drinking contaminated food or water. Children that live in orphanages are more prone to getting parasites like this because of the close living situations, the diapers, the care giver situations, etc. Most often the children live in close quarters with poor hygiene and there is little infection control.