**Typhoid is caused by Salmonella**

Typhoid fever is a bacterial infection that is caused by the bacterium Salmonella typhi. It is a highly contagious disease that spreads through contaminated food and water or through close contact with an infected person. Here is an explanation of why Salmonella is the correct answer:

**1. Introduction to Typhoid Fever:**
Typhoid fever is a severe systemic illness that primarily affects the gastrointestinal tract, causing symptoms such as high fever, abdominal pain, diarrhea, and headache. It is a significant public health concern in many developing countries, where inadequate sanitation and poor hygiene practices contribute to its transmission.

**2. Causative Agent:**
The bacterium Salmonella typhi is the causative agent of typhoid fever. Salmonella typhi is a gram-negative, rod-shaped bacterium that primarily infects humans. It is a member of the Enterobacteriaceae family, which also includes other pathogenic species such as Salmonella enteritidis and Salmonella typhimurium.

**3. Transmission:**
Typhoid fever spreads through the fecal-oral route, meaning that the bacteria are excreted in the feces of infected individuals and can contaminate food, water, or surfaces. The most common mode of transmission is the consumption of food or water contaminated with the bacteria. Additionally, direct person-to-person transmission can occur through close contact with an infected individual or by handling items contaminated with their feces.

**4. Pathogenesis:**
After ingestion, Salmonella typhi invades the intestinal mucosa and enters the bloodstream, leading to bacteremia. The bacteria then disseminate to various organs, including the liver, spleen, and bone marrow. This systemic infection causes the typical symptoms of typhoid fever.

**5. Diagnosis and Treatment:**
Diagnosis of typhoid fever is typically based on the isolation of Salmonella typhi from blood, stool, or urine samples. Antibiotic therapy is the mainstay of treatment, with appropriate antibiotics targeting the bacteria and reducing the severity and duration of symptoms.

In conclusion, typhoid fever is caused by the bacterium Salmonella typhi. Understanding the causative agent and its mode of transmission is crucial for implementing effective preventive measures, such as improved sanitation, safe food handling practices, and vaccination programs.