In treating cases of contagious disease the question often arises as to means of individual prophylaxis to be used by those who must be exposed to the effects of the infected locality or of the presence of the sick. The attempts which have been made to secure this individual protection in the midst of an epidemic have been numerous and varied, ranging from the use of the "vinegar of the four thieves" of the Middle Ages to the employment of the sulphites and chlorates to make the blood unsuited to the growth and multiplication of the supposed germs, or of cotton-wool respirators to strain the infected air, or of supposed specifics for particular diseases, as belladonna for scarlet fever and vaccination against small-pox. As yet, there is little or no satisfactory evidence as to the value of individual precautions against those diseases whose contagion is conveyed through the air, small-pox alone excepted, but in case of diphtheria in one member of a family of children it might be well to try the use of chlorate of potash internally, combined with the local application of the tincture of the chloride of iron, as suggested by E. M. Hunt. The question is one to be investigated by careful observation and experiment; and, though it is improbable that any definite results will be obtained except in those diseases which are communicable to animals, and therefore {207} susceptible of direct experiment, still, it is possible that some advance may be made. In rare and exceptional cases--as, for instance, in exploring a crowded, filthy, and intensely infected typhus-fever nest, as a tenement-house, or an infected yellow-fever ship--it may be worth while for the physician or inspector who is unprotected by a previous attack of these diseases to make use of a cotton-wool respirator, which is readily extemporized, and belongs to that exceedingly valuable and popular class of remedies which, "if they do no good, can do no harm." In epidemics of typhus, cholera, or yellow fever one of the most valuable prophylactics is to have a mind so occupied with other matters that it pays little or no attention to the danger, while in case of small-pox fear of the disease is indirectly the best prophylactic, since it leads to careful vaccination.