I'm given to understand the ubiquitous plague doctor mask is a later invention rather than existing in the 14th century.

In brief the theory of disease and infection at the time was that it was spread by 'Miasmas' and bad air and smells, the masks would be stuffed with spices and herbs to try and ofset this.

You can find more in depth answers from similar questions here:

/u/bedsiderounds provides probably the most comprehensive answer here https://www.reddit.com/r/AskHistorians/comments/94fwyc/during_the_time_of_the_black_plague_there_are_the/

While supplemental info can be found in answers here:

/u/reliable_information https://www.reddit.com/r/AskHistorians/comments/1b7tpp/how_might_the_historic_beak_nose_plague_masks/c94jvzh/

/u/staete https://www.reddit.com/r/AskHistorians/comments/vg2ll/were_the_traditional_beaked_plague_doctor/