Myth.
There is exactly one case where this may have occurred. And in that case, it is not clear if it was done intentionally. And that instance involved a British officer, prior to the American Revolution.
America gets a lot of criticism for wiping out Indians through disease and biological warfare. It wasn't us. It wasn't the United States of America. And most likely it wasn't anyone at all, at least as far as a deliberate policy was involved.
It's all just a myth created to make White people look bad.