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 Smallpox,
which is believed to have originated over 3000 years ago in India or Egypt, is one of the most
devastating diseases known to humanity. The causative agent, variola virus,
a member of the genus Orthopoxvirus, is relatively stable in the natural
environment.
The virus existed in more than one form,
some producing more severe illness than others. South
Asia was home to the more virulent strain of the disease,
variola major. Variola major could inflict a heavy loss of life amongst
non-immunized populations; killing 25 to 50 per cent of those infected. At
times it mutated into the deadly haemorrhagic form which caused rapid death
from cardiovascular collapse. The less virulent form, variola minor (or
alastrim) had much lower mortality rates, as low as 1 per cent, and was
prevalent in Europe and North Africa.
The disease was most infectious at the
appearance of the earliest lesions, the patient of smallpox remained
infectious to others mostly through droplet spread until the disappearance
of all scabs. More
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