(Circular.) To the physicians of Philadelphia, New-York, Baltimore, Norfolk and Newhaven : Gentlemen, as a malignant fever, has, for three summers past raged in different parts of the United States, and proved fatal to great numbers of our fellow-citizens ... it becomes highly important to take such efficacious steps as human wisdom can devise to prevent the introduction, arrest the progress or mitigate the severity of such a serious calamity.