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.