Conjure Wife by Fritz Leiber

Norman looked briefly across the campus before he went into Morton. The thing that startled him most was something he had not expected -- the air of normality. True, he had not looked forward -- at least consciously -- to any outwardly demoniac manifestations, any physical stench of evil. But this feeling of wholesomeness -- the little swarms of students trooping back to the dormitories or over to the campus soda fountain, the file of girls in white bound for a tennis lesson, the friendly smiles and nods, the way his own steps fell so easily and gratefully into the old familiar paths -- almost for a moment he wondered if everything else were not some crazy dream. It came to him almost with a shock that things were outwardly as normal as they had ever been, that only with respect to himself had they changed.

"Don't fool yourself," the voice inside told him. "Some of those laughing girls are already infected. Their very respectable mammas have given them delicate hints. Don't be too sure you know what they're thinking while they sip their cokes or chatter about their boy friends."