NEW YORK (AP) — Tiffany Pinckney remembers the fear when COVID-19 stole her breath. So when she recovered, the New York City mother became one of the country's first survivors to donate her blood to help treat other seriously ill patients.

"It is definitely overwhelming to know that in my blood, there may be answers," Pinckney told The Associated Press.

Doctors around the world are dusting off a century-old treatment for infections: Infusions of blood plasma teeming with immune molecules that helped survivors beat the new coronavirus. There's no proof it will work. But former patients in Houst...