Murray Leinster,
Ribbon in the sky
(1957)
“ But a different isolated group might contain a carrier of a different infection and be immune but distributive of it.It was literally true that each of the three cities might have developed in their first century of isolation a separate immunity to one disease and a separate defenselessness against all others. A member of one community might be actually deadly to a member of either of the others whom he met face to face.With icy wind blowing upon him as the sledge rushed on, Calhoun wryly realized that all this was wholly familiar. ”
