If you find a particular website (e.g. Internet Archive or the Library of Congress) difficult to search, try creating your own search engine for that site using Google.
Go to the Google search bar in your browser and enter your search terms (e.g. smallpox vaccination India).
Add the following to the end of your site: site:archive.org (where you replace "archive.org" with the website you want to search)
So your search terms would look like this:
smallpox vaccination India site:archive.org