Thyme contains carvacrol which is responsible for the antimicrobial properties. Other constituents of thyme help to relax the respiratory muscles and stimulate a wet or productive cough to encourage mucus expulsion.
This is a great immune boosting tea blend recipe containing this and other ingredients.
Tips for maintaing health while travelling are available here. Read this post to find out the difference between cold, flu's and allergies. For information about preventing allergies, check out this post. Tips for preventing colds and flu's can be found here and about preventing the spread of colds and flu's here.