It depends upon a specific contagion;—­occurs most frequently during childhood and adolescence, though no age is exempt from it;—­and affects the system but once; a peculiarity to which an exception is very rare, proved by the few instances of the kind which have been recorded.

The period at which the disease manifests itself after infection is various,—­generally about the ninth day; it has, however, been delayed until the sixteenth.

Description of the disease.—­The child infected will be observed not to be as well as usual, less active, and out of spirits; his appetite will fail, and his sleep be restless and disturbed.  It will soon be evident that he has apparently taken a cold in his head, and that this is accompanied by fever.  His voice will be hoarse; there will be frequent cough, headach, sneezing, running from the nose and eyes,—­the eyelids being somewhat swollen, and the eyes inflamed;—­the skin will be hot and dry, and he will complain of occasional chilliness.  In the course of the next two or three days, these symptoms will increase in severity, and perhaps be accompanied by oppression at the chest and hurried breathing, and towards evening by slight delirium.

On the fourth day, the rash will appear, but the symptoms will be little, if at all, mitigated; indeed, they will sometimes increase in severity.  The eruption will first be perceived about the head and face, in the form of small red spots, at first distinct from each other, but soon coalescing, and forming patches of an irregular crescent-like or semilunar figure, of a dull red colour, and slightly elevated (giving a sensation of hardness to the finger), while portions of the skin intervening between them will retain their natural appearance.  At this time the eruption will also be found on the inside of the mouth and throat, and the hoarseness will consequently increase.

On the fifth day, the rash usually covers the whole surface of the body, with the exception of the legs and feet; and is now very vivid on the face, which is not unfrequently so much swelled, especially the eyelids, that the eyes are quite closed up, as in small-pox.  On the sixth day, it is fully out on the extremities, and is beginning to fade on the face.  On the eighth, it is fading from all parts; on the ninth, it is hardly perceptible; and has entirely disappeared on the tenth day from the commencement of the fever, or the sixth from its own first appearance.  As the fading proceeds, the spots drop off in the form of little branny scales, which are sometimes, from their minuteness, scarcely perceptible.  They leave a slight discolouration on the skin, with considerable itching.

Such is the ordinary course of this disease; occasionally, however, deviations are met with.

Character of measles compared with scarlet fever and small-pox.—­Under the description given of Scarlet Fever, are noticed several signs by which that disease may be distinguished from measles:  to these may be added the absence of cough, of water flowing from the eyes, and of redness and swelling of the eyelids as in measles.  Again, in measles, the eruption is more pointed, of a crimson instead of a scarlet hue, and does not appear until two days later than in scarlet fever.