The paper discusses the spread of infections transmittable via droplet secretions generated during speaking, coughing, sneezing, etc. The main purpose of the paper is to define what happens to infectious droplets after leaving the body and entering the surrounding air. A simplified model for the analysis of droplet behaviour is constructed, with the force of gravit, Stokes’ drag and evaporation as the main factors affecting the droplets. A special emphasis is paid to the role of relative humidity of the air. From the model size dependent sedimentation time and evaporation time of droplets are derived. The model is then extended to include droplets of aqueous solutions, where the presence of solutes prevents the complete evaporation of droplets. The discussion is completed with an overview of important nonphysical factors that also affect the spread of droplet-transmitted infections but were left out of the presented model.

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