**1. Introduction**

Giardiasis is the intestinal infection caused by *Giardia lamblia* (also known as *Giardia intestinalis* or *Giardia duodenalis*). *G. lamblia* is a unicellular eukaryotic protozoan that was first described by Antonie van Leeuwenhoek in 1681—in his own stool sample [1]. For decades, it was considered of uncertain pathogenicity but now is recognized as common causes of diarrheal disease worldwide. Its clinical significance was broadly accepted after many symptomatic cases of giardiasis were diagnosed and reported among visitors to the Soviet Union in the early 1970s. Since then, giardiasis has been reported as responsible for many outbreaks throughout the world.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The epidemiology of giardiasis still is a matter of great discussion. From the original debates around its pathogenicity to the later ones about its speciation and biology, *G. lamblia* has proven to be an enigmatic and interesting organism [2]. Although giardiasis is currently recognized as one of the main causes of diarrheal disease and a leading cause of death and illness among children under 5 years old in developing countries [3], the long-term impact of pediatric giardiasis remains unclear. Recent cohort studies have confirmed a high prevalence of persistent, subclinical giardiasis and its association with growth shortfalls [4], but such evidence has not been consistently reported in the literature.

Commonly, giardiasis prevalence among poor populations is reported as very high, and when the infection became chronic, it has been associated also with malnutrition and cognitive deficits [5]. In developed countries, giardiasis represents the leading cause of traveler's diarrhea and is frequently reported among citizens that traveled to developing countries and expose themselves to untreated water from lakes, streams, and swimming pools [6–8]. These and other epidemiologic characteristics of giardiasis will be discussed in detail in this chapter based on the classical and latest literature.
