**3. Epidemiology**

Distributed worldwide, *Giardia* is probably the most frequent pathogenic intestinal protozoon in children and adults, and one of the most common nonviral causes of diarrhea, afflicting annually an approximate 280 million individuals. Due to its intensifying global burden and its developmental and socioeconomic impact on infected individuals, this parasitosis was incorporated in the Neglected Disease Initiative of the World Health Organization in 2004 [4, 7, 9, 10].

In developed countries, giardiasis is associated with social and climatic factors and is referred as a re-emerging infectious agent. Some epidemiological studies have shown that its prevalence varies between the population studied and the location, from 2 to 5% on industrialized countries to 20–30% in developing countries [1, 5, 11], and these changes in prevalence are associated, among others, with the hygiene infrastructure and the impact of the weather conditions, reason why environmental control efforts are necessary, which requires an integrated and systematic approach to decrease and mitigate the influence on the disease epidemiology; for this reason, it is linked to educational programs and other interventional measures [2, 6, 12].

Although in Latin America, a restricted quantity of population-based studies has been performed, in Cuba, according to the last national intestinal parasites survey (n = 5850), the prevalence of *Giardia lamblia* infection was determined to be at 6.02% (95%CI 5.40–6.63) [13].

This parasitosis is highly infectious where *Giardia* cysts are habitually excreted in considerable population, especially in young children after ingestion of contaminated water or food and through person-to-person contact. Cysts can survive for months in cold water, and they are relatively resistant to chlorination, reason why between 10 and 100 cysts are sufficient to establish infection 100% of the time. Consequently, ingestion of water or food that contains small levels of contamination can result in the disease, which is more recurrent in summer and fall. Other usual ways to transmit them could be among day care center attendees and people who live in residential institutions, and it could also spread by means of sexual activity, by oral-anal contact [1, 5, 11, 12].
