Coryzalia is a natural homeopathic remedy that can help stop a cold in its tracks, especially when taken at the onset of symptoms. Make sure to take 30 minutes away from strong tasting foods such as coffee or toothpaste as this can decrease the effectiveness. They also taste great and are safe for toddlers.
A natural homeopathic remedy that can help stop a flu is oscillococcinum.
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.