https://www.acf.org/our-community/news/far-good-blight-resistant-chestnuts/

In addition to the 15/16th hybrid Chinese/American sapling trees now being planted,
Naturally resistant “mutant” American Chestnut trees have been discovered in Kentucky and North Carolina,
These two are “one in ten billion” survivors of the fungus blight somehow.

The blight also infects but does not kill Oak trees, so the blight is kept alive in today’s US forests.