Transmission from one individual to another is never the essence of an epidemic; it may, in the form of 'miasma' or 'leaven', which can be communicated through water, food, contact, the wind, or confined air, constitute one of the causes of the epidemic, either direct or primary (when it is the sole, operant cause), or secondary (when, in a town or hospital, the miasma is the product of an epidemic disease caused by some other factor).