It is described as an obligate parasite with humans as its natural reservoir. Salmonella typhi is a gram-negative bacterium from the family Enterobacterioceae. It is a “multi-organ pathogen that inhabits the lymphatic tissues of the small intestine, liver, spleen, and bloodstream of infected humans” (Pollack, 2003). It does not usually inhabit or infect animals and the bacterium is most common in developing nations where sanitation is poor and there is limited supply of antibiotics. It is also described as a motile and facultative anaerobe which is very much susceptible to the action of antibiotics. There are about 107 strains of this bacterium which have already been isolated. Many of these strains have different metabolic characteristics and degrees of virulence. some of them are multi-drug resistant. Scientists studying this bacterium narrate that it contains the typical endotoxin expected of Gram-negative microorganisms as well as the Vi antigen which usually increases the virulence of the bacterium. It is also known to excrete ‘invasin’, a protein that lets non-phagocytic cells take the bacterium, and later enables it to live inside the cell. This ‘invasin’ sometimes also prevents the oxidative burst of leukocytes, thereby preventing the innate immune response to the bacterium (Pollack, 2003).
Typhoid fever is transmitted through the oral-fecal route. It is transmitted orally through food which is handled by an individual who frequently sheds the salmonella typhi bacterium through his stool or sometimes through his urine. The hand-to-mouth transmission is also possible “after using a contaminated toilet and neglecting hand hygiene” (Brusch, et.al., 2008). It may also be transmitted through sewage contaminated water ingested by humans. Shellfish which is taken from contaminated water, raw fruits and vegetables which are fertilized with contaminated sewage are also possible contaminants.