The Incidence of Salmonella Infections
How common are Salmonella infections and Salmonella outbreaks?
In 2009, over 40,000 cases of Salmonella (13.6 cases per 100,000 persons) were reported to the Centers for Disease Control and Prevention (CDC) by public health laboratories across the nation, representing a decrease of approximately 15% from the previous year, but a 4.2% increase since 1996.  Overall, the incidence of Salmonella in the United States has not significantly changed since 1996. [2, 5]
Only a small proportion of all Salmonella infections are diagnosed and reported to health departments.  It is estimated that for every reported case, there are approximately 38.6 undiagnosed infections.  The CDC estimates that 1.4 million cases, 15,000 hospitalizations, and 400 deaths are caused by Salmonella infections in the U.S. every year. 
Salmonella can be grouped into more than 2,400 serotypes.  The two most common serotypes in the U.S. are S. Typhimurium and S. Enteritidis. S. Typhi, the serotype that causes typhoid fever, is uncommon in the U.S.  But, globally, typhoid fever continues to be a significant problem, with an estimated 12-33 million cases occurring annually.  Moreover, outbreaks in developing countries have a high death-rate, especially when caused by strains of the bacterium that are resistant to antibiotic treatment. 
Salmonella are found in the intestinal tract of wild and domesticated animals and humans. Some serotypes of Salmonella, such as S. Typhi and S. Paratyphi are only found in humans.  For ease of discussion, it is generally useful to group Salmonellae into two broad categories: typhoidal, which includes S. Typhi and S. Paratyphi, and non-typhoidal, which includes all other serotypes.