He believed yellow fever was spread via contamination, so he arranged to have garments and a blanket that he believed were "infected" with the fever sent to the White House in hopes Lincoln would catch yellow fever. In reality, yellow fever is only spread through mosquitos.
I was so surprised to see this trivia question, because I was originally planning to discuss Dr. Blackburn in my TEDx speech, but since I only had 12 minutes, I had to delete Blackburn when I was cutting my speech down to size.