Small pox eradication was successful because of all except -

<p> Cross-resistance existed with animal pox. Epidemiological reason for smallpox eradication are No known animal reservoir, no long term carrier state,infection provides life long immunity,case detection simple due to characteristic rash,subcliical cases did not transmit the disease,a highly effective vaccine was available,internationalcooperatio. Reference:Park&;s textbook of preventive and social medicine,K.Park,23rd edition,page no:143. <\p>