Green Lantern #20 1963 Flash Crossover Hal Jordan Barry Allen Silver Age.
Story: When an illness that causes those infected to disappear, Green Lantern and the Flash are called in to help solve the mystery. As Carol helps out as a volunteer nurse and Iris West covers the story, Flash and Green Lantern monitor a man stricken with the illness. Green Lantern is unable to detect anything before the man disappears. Suddenly, Green Lantern becomes stricken with the illness as well.
Rushing Green Lantern to his power battery, he gives the Flash his power ring. As Flash recharges the power ring, Green Lantern vanishes and Flash is unable to do anything about it. Using a combination of scientific devices, his super speed, and the power ring, the Flash learns that the cause of the illness is a microscopic race of creatures.