**6. Epidemiology**

Epidemiology is the study of the distribution and causative agents of disease and the application of the study to the control of diseases. The epidemiology of salmonellosis cases differs extensively on the type of *Salmonella* spp. implicated. Annually, enteric fever was estimated to cause 200,000 deaths in 22 million illness cases, which mainly occurred in non-developed countries [45] and is low in developed countries. Enteric fever is endemic in many places on the African and Asian continents, as well as in countries throughout Europe, Central and South America and the Middle East. The prevalence and fatality rate caused by the enteric fever may vary greatly from one location to another. Enteric fever is uncommon in the USA and certain European nations, with less than 10 *Salmonella* cases reported per 100,000 people annually. The majority of cases reported in these nations are linked to travel, with foreigners or travellers returning from Pakistan, Africa or India, bringing the disease with them [45].

Contrary, non-typhoidal salmonellosis (NTS) incidence is increasing and continues to lead the gastroenteritis cases worldwide affecting 155,000 deaths from 93.8 million cases estimated every year [37]. Epidemiology data are well documented in developed countries, such as the USA and the countries of Europe, but are poorly compiled in less-developed countries like Asian and African countries. Owing to less effective monitoring systems, statistics on salmonellosis incidence are limited in countries of Asia, Africa and South and Central America, where only 1–10% of cases are reported [52–55]. It was reported that the most frequent serotype in Asia and Africa was *Salmonella* Enteriditis, accounting for 38% and 26% of the clinical isolates, respectively. NTS disease is an extremely serious infection in Vietnam, and the high death rate (26%) is comparable to the incidence in sub-Saharan Africa, which is a significant risk factor for both infection and mortality in HIV-infected individuals [54].
