Antigenic Stability Enables Smallpox Eradication

Steps:

• Identify pathogens: Smallpox from stable variola virus; TB from variable Mycobacterium tuberculosis bacterium.
• Compare vaccine efficacy: Smallpox vaccine induces lifelong immunity due to unchanging antigens.
• Assess eradication factors: Global campaigns succeeded for smallpox via consistent immunity; TB persists due to antigenic shifts.
• Conclude key difference: Low variation in smallpox antigens allowed uniform, effective vaccination.

Why C is correct:

• Antigenic variation refers to pathogen changes evading immunity; smallpox's minimal variation enabled one vaccine to confer durable protection, leading to eradication.

Why the others are wrong:

• A: Vaccines provide immunity for both (e.g., BCG for TB), but TB's variable antigens reduce effectiveness.
• B: Both diseases link to poor conditions, but this affects spread, not vaccine-driven eradication.
• D: Pathogen type (virus vs. bacterium) irrelevant; stable antigens, not classification, determine vaccine success.

Final answer: C