It is generally accepted that the infection of many diseases is conveyed by the discharge of droplets containing the biological materies morbi in the acts of coughing, speaking, etc. In some cases, for instance that of measles, it is believed that if a person harbouring the materies morbi has the opportunity of projecting it upon the upper respiratory tracts of non-immunes, a considerable proportion of the latter will develop the clinical disease. In other cases, for instance cerebro-spinal fever or acute poliomyelitis, it is believed that although, in consequence of the activity of the primary source of infection, many other persons will acquire and harbour the materies morbi only a small proportion of them will develop the disease.