Deaths resulting from influenza are commonly due to pneumonias resulting from an invasion of the lungs by one or more forms of streptococci, or by one or more forms of pneumococci, or by the so-called influenza bacillus, or bacillus of Pfeiffer. This invasion is apparently secondary to the initial attack. Evidence seems conclusive that the infective micro-organism or virus of influenza is given off from the nose and mouth of infected persons. It seems equally conclusive that it is taken in through the mouth or nose of the person who contracts the disease, and in no other way, except as a bare possibility through the eyes, by way of the conjunctivæ or tear ducts. PREVENTION. If it be admitted that influenza is spread solely through discharges from the noses and throats of infected persons finding their way into the noses and throats of other persons susceptible to the disease, then no matter what the causative organism or virus may ultimately be determined to be, preventive action logically follows the principles named below and, therefore, it is not necessary to wait for the discovery of the specific micro-organism or virus before taking such action. I. Break the channels of communication by which the infective agent passes from one person to another. II. Render persons exposed to infection immune, or at least more resistant, by the use of vaccines. III. Increase the natural resistance of persons exposed to the disease, by augmented healthfulness. _I. Breaking the channels of communication._ (a) By preventing droplet infection. The evidence offered indicates that this is of prime importance. (b) By sputum control. The evidence offered indicates that the danger here is due chiefly to contamination of the hands and common eating and drinking utensils. (c) By supervision of food and drink. Evidence offered does not indicate much danger of infection through these channels. Details and practical methods possible for the limitation of infection through droplets, sputum, and food and drink are discussed later under special preventive methods. _II. Immunization and vaccines._ (See the report of the laboratory committee appended.) In the present epidemic vaccines have been used to accomplish: 1. The prevention or mitigation of influenza _per se_. 2. The prevention or mitigation of complications recognized as due to the influenza bacillus or to various strains of streptococci and pneumococci.