This patient's most likely diagnosis is typhoid fever, also known as enteric fever. It is commonly caused by ingestion of the bacterium Salmonella enterica serotype Typhi. Infection is usually acquired via the fecal-oral route either from food or water handled by asymptomatic carriers or by introduction of the organism into sewage-contaminated water systems. Typhoid fever occurs throughout the world but is endemic in the underdeveloped areas of Africa, Asia, and Latin America. In the United States, infection is usually diagnosed in travelers who have come from the Indian subcontinent. The typical clinical presentation of typhoid fever is a progressively rising fever, accompanied by abdominal pain, initial constipation followed by diarrhea, and relative bradycardia. One third of patients develop salmon-colored blanching maculopapular lesions on the trunk or abdominal wall. Tender hepatosplenomegaly is a common finding. Abnormal laboratory results often include anemia, leukopenia with a relative lymphopenia, thrombocytopenia, and elevated liver enzymes and bilirubin levels. The diagnosis may be supported by serologic assays. However, isolation of the organism from blood, stool, urine, and/or bone marrow is required for confirmation.

Brucellosis, another zoonotic infection mostly caused by one of four species named for a specific animal reservoir, occurs through direct contact with infected animals, ingestion of infected animal products, or inhalation. Fever occurs and is often intermittent or undulant. Arthralgia and arthritis, neurologic and psychiatric symptoms, and genitourinary involvement are also common. Gastrointestinal symptoms are infrequent and nonspecific.

Lassa fever is caused by an arenavirus endemic to Western Africa. Infection can be acquired after direct or indirect contact with infected rodent excreta. High fever, abdominal pain, and headache may progress to hemorrhagic manifestations and death.

Leptospirosis is a common zoonotic infection caused by spirochetes found throughout the world. Human infection often follows occupational (farmers, abattoir workers, veterinarians) exposure of damaged skin or mucous membranes to body fluids or environmental sources that have been contaminated with urine from an actively infected animal. However, acquisition of infection related to travel and recreational activities is becoming more common. Disease manifestations include acute fever, muscle pain (typically lumbar and calf regions), redness of the conjunctiva, and, occasionally, aseptic meningitis. Gastrointestinal symptoms are infrequent. Progression to a more severe disease phase of consisting of multisystem damage is rare.