Oh, that reminds me about the diphtheria thing.
So how is it that in the US diphtheria incidence and deaths dropped steadily before the intro of the vaccine and in England the drop began exactly at the same time that the vaccine was introduced. How convenient!
However, if you look closely at the graph, there was a dramatic drop right around 1939 and then it started going up again, even after the vaccine began to be used and then there was a dramatic downturn. So it isn't actually as obvious as it looks at first glance.
Here are some of the other factors at play in the UK.
During World War II the English evacuated many children from the city slums into the countryside. This was undoubtedly traumatic for the children, but it was also a big change in living conditions for most of them, with better food, more food and for some, their first experience of indoor plumbing and other amenities. Then there was a long stretch without much going on and many of the children returned to their families in the cities, where they returned to inadequate food and inadequate sanitation.
However, two other factors were at play which would have had an effect on the health of children. One was rationing, which favored children, even poor children and provided extra milk, fruit and eggs for children and pregnant women. The second was high employment, which meant that there were funds available to purchase the little food which was available. So, the vaccine was not the only factors at play. Incidentally, rationing continued until 1954, and with the introduction of some degree of socialism in the UK, the living conditions never slid back to the pre-war misery that allowed illnesses like diphtheria to run rampant in the slums of big cities.
Here is a link to an article on rationing http://www.primaryhomeworkhelp.co.uk/war/rationing.htm
Here is one on the evacuations http://www.tnunn.ndo.co.uk/evacuees.htm
which does point out that sometimes it was middle class children from cities ending up in poor villages -- lots of culture shock all the way around!