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Quite a few years ago I read a book that I think was called The Mustard Seed. It was a small paperback, perhaps 200 pages at most. Probably from the 1960-early 70s.
The book involved a mass epidemic of influenza that nearly everyone was dying from. The only "cure' was a mustard plaster applied to the chest of the victim. One man figured this out and went around saving his friends and family.
I'd love to read this book again, but every "mustard seed" book I find anywhere, including Abebooks is a christian guidebook.
I was able to do a search on Amazon for the title just the the SF/Fantsdy genre and it was not listed. Perhaps the title was slightly different. Try ABEbooks, in their search forum called Booksleuth in the Community area. Some of the people there are amazing. Give them the description you gave us and I bet they could help you.