A new tick-borne disease that may be stealthily infecting some Americans has been discovered by Yale researchers working with Russian scientists. The disease is caused by a spirochete bacterium called Borrelia miyamotoi, which is distantly related to Borrelia burgdorferi, the spirochete that causes Lyme disease. B. miyamotoi has been found — albeit relatively rarely — in the same deer tick species that transmit Lyme, and the Yale researchers estimate that perhaps 3,000 Americans a year pick it up from tick bites, compared with about 25,000 who get Lyme disease. But there is no diagnostic test for it in this country,...