Antibodies capable of neutralizing human influenza virus were present in the sera of old swine on two New Jersey institution farms, but absent from the sera of young swine on the same farms. The old animals had lived through the winter of 1936–37 in which outbreaks of upper respiratory tract disease were prevalent among the human inmates of the two institutions, while the young swine studied were born long after these outbreaks. It is believed that the swine whose sera neutralized human influenza virus had undergone an unrecognized human influenza virus infection acquired from man. The possible bearing of these observations upon the theory that swine influenza was originally of human origin is discussed.

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