When considering how people are infected by viruses or bacteria and what can be done to prevent the infections, answers from many disciplines are sought: microbiology, study of pathogenesis, epidemiology, medicine, engineering and sometimes physics. There are many pathways to infection spread, and among the most significant is airborne transport. Microorganisms can become airborne when aerosol droplets are generated and released during speech, coughing, sneezing, vomiting, or atomisation of faeces during sewage removal and treatment. The fate of the droplets in the air is governed by the physical principles of transport, with droplet size being the most important factor affecting the distance travelled by droplets immediately after generation, their dispersion and deposition on surfaces. Droplet size is also the key factor determining the survival of microorganisms within the droplets. In addition to the droplets’ physical properties, physical characteristics of the indoor environment such as temperature, humidity, and air flow characteristics, as well as the design and operation of building ventilation and filtration systems; are of critical importance in affecting indoor infection spread. Do we understand the mechanisms of infection spread and can we quantify the droplet dynamics in the air under various indoor environmental conditions? Unfortunately no, as this aspect of infection spread has attracted surprisingly little scientific interest. However, investigations of numerous cases in which a large number of people were infected with a serious disease show how critical the physics of microorganism spread can be. This paper reviews the state of knowledge regarding the mechanisms of droplet spread in indoor environments and the solutions available to minimize the spread and prevent infections.