It is important to remember that advances in medical science only lead to improvements in health to the extent that they are generally applied. . . .[In Britain, for example,] there was a dramatic improvement [in the deaths of children from diphtheria] in 1941; an effective diphtheria vaccine had been commercially available from about 1922, but was not applied as an organized programme for mass immunization to the whole child population until nearly 20 years later.