rot implies wood,,old heavy wooden oak doors, if one corner or surface is a bit falling out,can be filled with wood putty,water based wood putty,colored to match at the home depot paint mixing station
water is the number one wood rotter,keep it away from the door area so it runs off,
a great wood preservative and sealer,,insect repelling is linseed oil mixed with turpentine,,,,,, old heavy wood doors are usually prized as artifacts
a plastic sheet is useless.
post a pic of the door & the threshold,might be informative to the problem.