It has been found that in most parts of the United States from 1 to 2 per cent of the population suffer from hay fever at some portion of the year, indicating that its victims may be counted by the hundred thousands. It was not until about 1870 that the relationship of pollen to the disease was established. The development of hay fever about the time of hay harvest led to the belief that this was the cause of the disease. The particular pollens causing this disease were for a long time unidentified.