**Core Concept**
Shigellosis is a bacterial infection caused by the Shigella species, primarily affecting the small intestine. The pathogenesis involves the invasion of intestinal epithelial cells, leading to inflammation and fluid loss.

**Why the Correct Answer is Right**
The correct answer is **A**: fecal-oral transmission. This mode of transmission occurs when contaminated food, water, or surfaces are ingested, allowing the bacteria to enter the body. In developing countries, inadequate sanitation and hygiene contribute to the spread of Shigella. The bacteria can survive for extended periods on surfaces, making it a common mode of transmission in areas with poor sanitation.

**Why Each Wrong Option is Incorrect**
**Option B:** Waterborne transmission is a possible route, but it is not the most common mode of acquiring Shigella infection. While contaminated water can be a vehicle for transmission, the primary route is through fecal-oral contact.
**Option C:** Person-to-person transmission does occur, particularly in crowded areas, but it is not the primary mode of acquiring Shigella infection.
**Option D:** Vector-borne transmission is not a known mode of acquiring Shigella infection.

**Clinical Pearl / High-Yield Fact**
Shigella infection can cause severe diarrhea, often with blood and mucus, leading to dehydration and electrolyte imbalances. Prompt recognition and treatment are essential to prevent complications.

**Correct Answer: A. Fecal-oral transmission**