Although most of the infections of the respiratory tract are mild, self-limited illnesses, they are ever-present and the morbidity that they cause is very high. In developing countries the case fatality rate is still significant; and whereas the rate is considered low in developed countries, even there the number of deaths – especially in children and in the older age groups – is high, because of the size of the illness pool.
In the last decade, communicable diseases of the respiratory system, as a group, were one of the principal causes of morbidity and mortality in many countries. Data reported by 88 countries with a total population of about 1200 million show that, in the year 1972 alone, more than 666,000 deaths were related to acute respiratory infections. This represents an average of 6.3% of all deaths reported, although there are considerable differences between continents and between countries, with an overall range of from 3.0% to 13.6%. Pneumonia, both viral and bacterial, accounted for 75.5% of the deaths related to acute respiratory infections.
For children in developing countries the situation is particularly alarming. The mortality rate may be more than 50 times that in developed countries. These were the conclusions of WHO, drawn from analyses of mortality and case fatality from acute respiratory infections in children under five years of age in nearly all the countries of the Americas and Southeast Asia and in several countries in the African and the western Pacific. It is a result of low birth weight, very young ages of mothers and births following too closely upon one another, malnutrition, lack of breast-feeding, indoor air pollution stemming from burning of 'biomass' fuel and exposure to 'passive smoking' at home.