Dear Professor White,
Many thanks for your letter dated 15th August, inviting me to attend and chair a session of the forthcoming 20xx International Conference on Parallel Data Processing to be held in Bellaire, Michigan, from October 25 to 28, 20xx.
Much to my regret, I shall not be able to honor the invitation because I have been suffering from a disease since this summer. I am firmly advised that it would be unwise to undertake any distant and long travel in the near future.
I feel very sad to miss the opportunity of meeting you and many others in the field of Computer Science. I wish the conference a complete success.
Faithfully
yours,
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