I only read it this year but I think it could have been written in the last 4 or 5 years. It's seen through two different time periods. One is the far past where an explorer or maybe a missionary is staying with these tribal/primitive people that live in Nepal? Or some far off region. Although it is two tribes they are separated by blood lines. One wears a yellow belt and the other blue and depending on which belt color you have you can only marry and procreate within your color or else the natives feel a demon will be released.
Then in the future, there is a plague and two scientists search for the clues even using the historical documents to find patient zero who had cross-bred with the opposite belt people. It is only after the wife loses her husband and child and many years later that she finds the cure.