This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
Smallpox eradication was successful due to all of the following reasons except 
Select and explain. 
A. Subclinical cases did not transmit the disease, B. A highly effective vaccine was available, C. Cross-resistance existed with animal pox, D. Infection provided lifelong immunity
C: Cross-resistance existed with animal pox
Answer: C: Cross-resistance existed with animal pox 
Explanation: Epidemiological Reasons/Basis For Smallpox Eradication:
• No known animal reservoir
• No long term carrier state
• Infection provides lifelong immunity
• Case detection simple due to characteristic rash
• Subclinical cases did not transmit the disease
• A highly effective vaccine was available
• International cooperation