It is enough to think of those studies in bacteriology which refer to the vehicles of infectious maladies in the environment, in order to recognize therefrom a primary token of the important place which is assigned to the community of human interests, and this is now affirmed with an emphasis never before displayed. Microbes multiply chiefly in damp and dirty places; underfed people are more prone to illness than others, and so are those who are overtired. Therefore illness and early death must be the heritage of the poor who, underfed and overtired, live in damp and dirty places? No. It is a question of vehicles. Microbes spread in all directions from the sources of infection, by means of dust, insects and all the usual objects of life, in fact by all the means of transport. They exist in inconceivable and fabulous numbers; and every sick person is an almost incredible source of illness and death. One single person would suffice to contaminate the whole of Europe. The means of transport allow microbes to cross oceans and continents in every sense. We need only observe the transatlantic lines, and those of the railways of the world, in order to realize the lines of communication between the maladies which afflict humanity in all the places of the earth. We need only study the industrial changes of matter in order to follow in detail the daily path of the microbes, which put all classes of society into intimate communication. The rich lady wears linen on her person which comes from the hands of the poor, and is constantly in their keeping; she cannot put food into her mouth unless it is offered to her by the poor who have handled it over and over again. The air which is breathed by the rich may contain in its dust the desiccated germs which a consumptive workman has scattered on the ground. There is no way of escape. Statistics prove this: the death rate from infectious diseases is tremendously high in all countries, among both rich and poor, although the poor die in a double proportion to the rich. How can we deliver ourselves from this scourge? Only on condition that there be no more sources of infection, that is to say, that there be no longer unhealthful places in the world, and no underfed people constrained to work beyond their strength. The only way by which the individual may escape is that by which all humanity may be saved. This is a great principle, which seems to ring like a trumpet call: Men, help one another, or you will die.