The decline in infectious disease mortality in England and Wales beginning about 1880 has been attributed to improved nutrition, hygiene, and sanitation. Such an explanation does not adequately explain the lack of improvement in infant and diarrheal disease mortality before 1900 nor the abrupt subsequent decline. A hypothesis was proposed that the decline in fertility rate was a major cause of the decline in infant mortality by raising the median age at infection. The hypothesis could only be tested indirectly. A review of morbidity data demonstrates the importance of family characteristics on the median age at infection for measles, pertussis, and common respiratory illness. The association of parity with infectious disease mortality supports the hypothesis. A method was developed for estimating the change in birth order distribution resulting from declining fertility. Using 1949-1950 data, it was shown that declining fertility could account for at least a 24% decline in postneonatal mortality due to bronchitis and pneumonia. Age-specific measles mortality rates are consistent, with an increase in age at infection. Declining fertility appears to have played a major role in the decline in infectious disease mortality in England and Wales by increasing the median age at infection.