This chart shows age-standardized mortality rates (mortality rates adjusted for the differing proportions of people in different age groups in the population) from important infectious diseases (TB, tuberculosis; B,P,I, bronchitis, pneumonia and influenza) over the period 1851-1930.

What does the graph show? ( Note: the scale is logarithmic.)

The most obvious feature is the decline in mortality from these diseases over the period covered. To what can we attribute this decline?

It is difficult to explain this decline by reference to medical technology for the simple reason that there was no medical technology for most of this time. Indeed, the causative agents for all of these conditions were unknown at the time the declines began: immunizations were not introduced until after the period covered by the chart; antibiotics did not exist and the ideas of antisepsis and hygiene emerged only in the 20th century. So what was responsible for these declines?

A second important question is who did these diseases tend to kill? What effect would a decline in deaths among the most vulnerable groups have on expected years of life at birth?