The transmission of communicable diseases is highest in urban slums. Childhood infections are not seasonal but all year round phenomena. In spite of the availability of of preventive measures, immunization coverage remains low in urban areas, especially in developing countries. Easier access to services, fewer logistical problems for supplies and exposure to communication media have not made ae difference to the well-being of children in slums.
Urbanization can be an important source of health problems: nearly half of the world's population will live in urban centres by the end of the 20th century but currently 30-60 per cent of the urban population are in low-income countries, and lack adequate housing with sanitary facilities, drainage systems, and piping for clean water (UNCHS 1996). This number is expected to increase since local and city authorities often lack the resources, knowledge, trained personnel and financial capacity needed to meet their responsibilities in providing services and amenities essential for healthy living. Increased exposure to biological and chemical health risks in urban areas is particularly harmful to children. Children suffer the greatest number of deaths due to diarrhoeal diseases (2.5 million deaths per year) and this number is likely to increase in populations of the urban poor. Prevalence of asthma, often exacerbated by air pollutants, has also increased among children (Woolcock and Peat 1997). Studies suggest a quantitative relationship between atmospheric carcinogen levels and lung cancer, and WHO has estimated that 50 per cent of the global burden of chronic respiratory illnesses is associated with air pollutants (WHO 1997b).