Something else is at work: exposure to infection. Repeated infection during infancy and childhood slows growth as nutrition intake declines or is used by the body to fight disease. Predominant among these illnesses are respiratory infections, notably pneumonia and bronchitis, and gastro-intestinal infections, especially diarrhoea and dysentery.

The key factor here is the urban environment. Sanitary reforms improved the quality of water supply and the disposal and treatment of sewage. In urban districts horses disappeared from the streets and pigs from backyards. Equally important was the reduction in overcrowding and improvement in the quality of housing, as the slums were gradually cleared.