Final answer:
Typhoid Fever is caused by S. typhi, which can persist in a carrier state within the gallbladder or intestinal epithelium of asymptomatic carriers. The bacteria's pathogenicity islands and virulence factors help it to avoid the immune response and establish a carrier state, allowing for transmission to others.
Step-by-step explanation:
The germ that causes Typhoid Fever can enter and remain in a carrier state because some individuals carry S. typhi without presenting signs or symptoms (asymptomatic carriers) and continually shed the bacteria through their feces. These carriers often harbor the bacteria in the gallbladder or intestinal epithelium.
Moreover, S. typhi have Salmonella pathogenicity islands (SPIS) that contain the genes for many of their virulence factors, which helps them survive and reproduce within the host, even without causing symptoms. The presence of the Vi antigen and chimeric A2B5 toxin, among others, contributes to the pathogen's ability to evade the host's immune system and establish a carrier state. Because of this, individuals can unknowingly transmit the disease to others, as in the case of 'Typhoid Mary', who was an active carrier of the disease.