This is, strictly defined, conveyance of disease by touch or contact. But some (not all) disorders, which may be transmitted by actual touch, pass also to a short distance through the air. This is true of typhus, small-pox, chicken-pox, measles, scarlet fever, mumps, and whooping-cough, certainly; perhaps, in rare instances, of diphtheria. Hydrophobia, syphilis, and gonorrhoea are conveyed only by contact and inoculation; that is, introduction of the virus of the disease into the blood, or, at least, under the skin. These diseases, are, in fact, the common diseases that are certainly contagious.