B., a boy aged 16, slept in the same bed with his brother, C., aged 14, who had scarlet fever. B., therefore, was in constant contact with his brother for two nights preceding the appearance of a typical rash, and during the usual onset with severe sore throat and high temperature. On diagnosis, the morning of the third day, B. was kept out of his sick brother's room, but remained in the same house. Eleven days after the appearance of the rash, C. left his room and associated with the family and with B. The same day an uncle, aged 35, visited in the home. Ten days after this, B. and the uncle both developed sore throat and three days later the typical rash of scarlet fever. It seems probable that B., after being thoroughly exposed during the first three days of his brother's infection with scarlet fever, did not