This generation, which is so thankless to the great discoverers of the causes of disease, so forgetful of the epoch-making labours of the English sanitary reformers of last century, has not seen nor even heard of the awful thing once known as "gaol-fever." A hundred years ago it was as dangerous to the life of an unhappy prisoner to await his trial in Newgate as to stand between the opposing forces on a battlefield. Gaol-fever attacked not only the prisoners, but the judge and the jury and the strangers in the court. The aromatic herbs with which the hall of justice was strewn were supposed to arrest the spread of the terrible infection, and it is still customary to provide with a bouquet of such plants the judge who presides at a "gaol delivery." The inexorable ministers of justice, who, seated high above the common herd, and clad in their ancient robes of office, were about to deal shameful death to the guilty wretches brought from the prison cells, were often themselves struck down by the Angel of Death moving invisibly through the court. The "black assizes" were not isolated, but repeated occurrences in our great cities. Typhus fever was the name given by the learned to this awful pestilence. There was a mystery and horror surrounding it which paralysed those who came into contact with it, and produced something like consternation. Men fled in terror from the infected buildings, business was arrested, the universities deserted, palaces left empty, and the dying abandoned to their misery when it appeared. There was a feeling that some deadly unseen power was present, irresistible and malignant. It is only to-day--in fact, within the last two years--that we have learnt what that unseen power was. The Angel of Death which moved through the Old Bailey Sessions House in bygone days was, indeed, a living thing. It passed silently and unseen from the prisoner to the warder, from him to the usher, thence to the bar--the jury and the exalted judge. It had no wings, yet it moved slowly and surely carrying black death with it. This terrible and mysterious assassin has at last been unveiled. The shroud of concealment has been torn away and there the dire monster stands--naked, remorseless and hideous. It is of small size, though it makes us all shrink with horror and disgust. It has six claw-like legs and no wings. It is, in fact, neither more nor less than the clothes louse, the _Pediculus vestimenti_. The filthy, crowded condition in which the prisoners were kept, and (let us well remember and reflect thereon) the personal want of cleanliness of judge, jury, barristers and ushers, rendered the existence of the little parasite and its effective transference from man to man possible. Those pompous emblems of authority, the horsehair wigs--those musty robes of unctuous dignity--were full of dirt, and harboured the wandering bearer of typhus infection. Gaol-fever was due to dirt; its infecting germs were distributed by loathsome insects.