D. The hundred-thousands place.
Very helpful in the long run. However it could be harmful to some animals
Smallpox<span> vaccine, the first successful vaccine to be developed, was introduced by Edward Jenner in 1796. He followed up his observation that milkmaids who had previously caught </span>cowpox<span> did not later catch </span>smallpox<span> by showing that inoculated</span>cowpox<span> protected against inoculated </span>smallpox<span>.</span>
