Leprosy cannot be eradicated because:
|A||Highly infectious but low pathogenecity|
|B||No effective vaccine|
|C||Only human reservoir|
|D||Long incubation period|
Although both b & d are correct options the best answer to go for would be d.
Factors which help in disease eradication are as follows:
1. No known animal reservoir
2. No long-term carrier of the virus
3. Life-long immunity, after recovery from the disease
4. The detection of cases comparatively simple because the rash was so characteristic and occurred in visible parts of the body.
5. Persons with subclinical infection did not transmit the disease
6. Vaccine highly effective; easily administered, heat stable and confers long-term protection.
7. International cooperation
Leprosy falls far short of these requirements and, therefore, is not a suitable candidate for eradication. The long and variable incubation period, the disputed modes of transmission, the presence of subclinical cases and our inability to detect them in the community, the complicated spectrum of disease manifestations, the failure of the cell mediated immunity in lepromatous cases, bacterial resistance and persistence in the human body, absence of a vaccine, social and cultural taboos leading to concealment of the disease and the recent discovery of extrahuman reservoirs are all real hurdles which smallpox did not pose.