Clinical measles is rarely diagnosed in an individual who has a history of previous infection. In a survey1 of more than 1,000 pediatricians whose collective experience embraced many hundreds of thousands of patients, only 7 instances of second attacks of measles were disclosed. This is the more remarkable when considered in the light of current knowledge of the occurrence of exanthemata associated with a number of the newly discovered virus infections which afflict children.
One of the commonly offered explanations for the persistence of functional immunity to various childhood infections is the periodic stimulation of repeated exposure. However, that such a mechanism is necessary for the maintenance of measles immunity has appeared improbable ever since the classical study in the Faroe Islands by Panum. It was pointed out by Panum2 that almost the only ones to escape infection in the 1846 epidemic were those 98 oldsters who had