The PLAGUE was formerly universally regarded as a contagious disease, and to prevent the horrors which attend its introduction in large cities, the most stringent laws have been enacted for ages. But the contagiousness of the plague is now doubted by many enlightened physicians. Whether it be so or not, it never made its appearance in countries bordering on the North Sea or the Baltic, or on the American continent. Although many vessels every year, almost every month, arrive in our principal ports from the Levant, freighted with rags and other articles, constituting a medium through which this disease, if contagious, would surely be propagated, yet this dreadful scourge of cities, in ancient and modern times, has never been brought across the Atlantic. The small pox is another disease against the introduction of which quarantine laws have been established. That it is contagious there is no question; but by the blessed discovery of vaccination, this disease, once so dreadful, is robbed of its horrors, and rendered as harmless as the measles or the whooping cough, insomuch that laws, formerly enacted in different states to protect the people from the dangers of the small pox have generally been repealed. The Asiatic cholera, when it first made its appearance in Europe, was believed to be contagious. Quarantine laws, of the most stringent character, were adopted to prevent its introduction into seaports, and military CORDONS SANITAIRE were drawn around the frontiers of nations to shut it out of villages and towns, until it was ascertained to be an epidemic disease, the germs of which were in the atmosphere, and could no more be controlled than the winds which sweep the earth. The YELLOW FEVER, however, has for many years been the most terrible bugbear, and to prevent its introduction into the seaports of Europe and the United States has been the chief end and aim of the absurd and ridiculous quarantine regulations to which I have referred. It has never been regarded as contagious by well-informed men in countries where it is most prevalent, and now, in spite of long-existing and deeply-stamped prejudices, it is generally admitted, by enlightened physicians, that the YELLOW FEVER IS NOT CONTAGIOUS. NOT A SINGLE WELL-ESTABLISHED FACT CAN BE ADDUCED TO SHOW THE CONTAGIOUS CHARACTER OF THE DISEASE, OR THAT IT CAN BE CONVEYED IN CARGOES OF ANY DESCRIPTION FROM ONE COUNTRY TO ANOTHER. Persons in good health may leave a port where yellow fever prevails, and carry within them the seeds of the disease, and on arriving at another port several days afterwards, or on the passage thither, may be attacked with the disease in its most appalling character, and die; BUT THE DISEASE IS NOT COMMUNICATED TO OTHERS. Indeed, the yellow fever is not so INFECTIOUS as the typhus or scarlet fever, which prevails every season in northern climes.