Epidemiologically important strains of MDRST started emerging in the late 1980s. The number of reported MDRTF cases rapidly increased throughout the world from 1989 onwards with most of the cases being trom Southeast Asia and the Middie East region. Today the world can be categorized into three different zones based on the geographic distribution of MDRST strains.

The "endemic zone" includes Southeast Asia, Africa, Latin America and parts of China. MDRST are now endemic in many of these areas, especially in the Indian subcontinent. Frequently protracted MDRTF epidemics have been reported from this endemic zone since 1989. These epidemics vary in extent from localized outbreaks in a city to widespread outbreaks involving many cities, states, and even a whole region. Currently the incidence of MDRST strains reported in these studies from the Indian subcontinent and China ranges between 50% and 80; of all the S. typhia isolates, and it is close to 100% during the out breaks. In the absence of any reliable epidemiologic data the true incidence of MDRST in the endemic zone remains unknown.

The "pseudo-endemic zone" which encompasses the Middle East region, particularly the Gulf countries, where endemicity of MDRST strains presents a unique problem. About one-third to one-half of the population in the Persian Gulf are expatriate workers, mainly from the Indian subcontinent and Far East region. These workers travel back to their native countries during vacations where they may be exposed to S. typhia. According to recent reports from Bahrain, Kuwait and Qatar, 70% to 80% of cases of typhoid fever were imported. The incidence of MDRST in these studies ranges between 5% and 30%.

The "nonendemic zone" includes all the developed countries of the world, where most cases of MDRTF are among international travelers. Because of the high standards of hygiene, chances for the spread of MDRST in the nonendemic zone are negligible, affected in an endemic area and constitute about 40% to 50% of the cases of MDRTF during outbreaks. Of these, 15% to 50% are below the age of 5 years. This is because of the known association of typhoid fever outbreaks with malnutrition, which is rampant in children under the age of 5 years in developing countries. It is possible that malnutrition enhances susceptibility to typhoid infection by alterations in the intestinal flora or other host defenses. Other risk factors for young children may be their unhygienic habits and their dependence for food on adults, who may be the carriers. Compared with the children infected by susceptible S. typhia strains, children with MDRTF are sicker and more toxic at admission with a significantly higher incidence of life-threatening complications such as endotoxic shock, encephalopathy, myocarditis and paralytic ileus.

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