Smallpox eradication was declared by WHO in 1980. All of the following epidemiological factors are have a role in eradication of smallpox, EXCEPT:

Epidemiological factors which led to the eradication of Smallpox: No known animal reservoir No long-term carrier of the virus Life-long immunity, after recovery from the disease The detection of cases comparatively simple because the rash was so characteristic and occurred in visible pas of the body. Persons with subclinical infection did not transmit the disease Vaccine highly infective; easily administered, heat stable and confers long-term protection. International cooperation Ref: Park's textbook of Preventive and Social Medicine, 20th edition, Page 131.