The International Medieval Bibliography is the best database to use when looking for academic journal articles and book chapters in the field of medieval European history.
Link your search terms using the AND connector. For example: leprosy and england. On the search results screen, use the options on the left toolbar to narrow your search results. To find the fulltext of the article (if not provided directly in the database), click on the "Check for Full Text" button and work your way through the screens. Not all articles will be online, some may only be in print, others may not be available at the UW. Articles from journals which the UW does not own can be requested via Interlibrary loan.