## **Core Concept**
The infective dose of a pathogen refers to the minimum number of microorganisms required to cause infection in a host. Shigella, a genus of Gram-negative bacteria, is known for causing shigellosis, a diarrheal disease. The infective dose of Shigella is relatively low due to its high virulence and ability to survive passage through the stomach.

## **Why the Correct Answer is Right**
The correct answer, , indicates that the infective dose of Shigella is as low as 10-100 organisms. This low infective dose is attributed to Shigella's ability to invade intestinal epithelial cells and evade the host's immune response effectively. The low dose required for infection highlights the high virulence of Shigella and its potential for causing outbreaks, especially in areas with poor sanitation.

## **Why Each Wrong Option is Incorrect**
- **Option A:** . This option suggests a much higher infective dose than what is established for Shigella, which does not align with the known high virulence and low infective dose of the bacteria.
- **Option B:** . Although closer, this still overestimates the infective dose of Shigella, which is known to be much lower.
- **Option D:** . This option suggests an even higher infective dose than Option A, which is not consistent with the characteristics of Shigella infections.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that Shigella has a very low infective dose, making it highly contagious and capable of causing significant outbreaks, especially in crowded settings or areas with inadequate hygiene and sanitation. This low infective dose also underscores the importance of strict infection control measures.

## **Correct Answer:** . 10-100 organisms.