Airborne diseases (tuberculosis, pneumonia, diphtheria, bronchitis, whooping cough, meningitis, influenza, measles, chicken pox) spread by breathing in the airborne respiratory secretions of infected persons, represent the second major cause of morbidity in developing countries. Several million persons per year die of acute respiratory diseases. Together with diarrhoea, this is the most common cause of death of children under five in developing countries.