Why is a low infectious dose a notable feature for Shigella and Campylobacter infections?

• A. Small inoculum suffices to cause disease, enabling rapid person-to-person spread and outbreaks
• B. Large inoculum is required for disease
• C. Only affects immunocompromised individuals
• D. They are resistant to environmental stress

Answer: (A)

Infectious dose is the smallest number of organisms needed to establish infection in most people, and Shigella and Campylobacter infections are notable for their low infectious dose. This means only a few bacteria can cause disease, which makes transmission from one person to another more efficient and helps spark outbreaks. Shigella is highly contagious via the fecal-oral route, so even tiny amounts in contaminated food, water, or on hands can initiate infection and rapid spread, especially in crowded settings. Campylobacter can also cause illness from relatively small exposures, often linked to undercooked poultry or contaminated food, enabling outbreaks through common food-handling and consumption practices.

The other ideas don’t fit because requiring a large inoculum would imply much less person-to-person spread, and these pathogens can cause disease in otherwise healthy individuals, not only the immunocompromised. They also aren’t defined by strong environmental resistance like spore-formers; transmission is driven by the low dose and fecal-oral spread rather than resilience in the environment.