We protect ourselves from airborne cross-infection in the indoor environment by supplying fresh air to the room by natural or mechanical ventilation. The air is distributed in the room according to different principles as e.g. mixing ventilation, downward ventilation, displacement ventilation, etc. A large amount of air is supplied to the room to ensure dilution of airborne infection. The talk discusses both the macroenvironment and the microenvironment. The macroenvironment is the conditions created by the air distribution system, and the microenvironment is the conditions created by the local flow around persons in combination with the surrounding conditions. Analyses of the flow in the room (macroenvironment) show that there are a number of parameters that play an important role in minimising of airborne cross-infection. The air flow rate to the room must be high, and the air distribution pattern can be designed to have a high ventilation effectiveness. Furthermore, personalized ventilation may reduce the risk of cross-infection, and in some cases it can also reduce the source of infection. Personalized ventilation can especially be used in hospital wards, aircraft cabins and, in general, where people are located at defined positions. Analyses of the flow in the microenvironment show that a number of variables are important, as e.g. distance between people, people’s posture, surrounding temperature gradients and surrounding temperature, activity level, etc. Experiments with tracer gas simulating droplet nuclei and experiments with large particles, simulating droplets are used for the study of airborne cross-infection risk, and for the study of droplet-borne transmission of a disease. CFD predictions are used to support the experiments and the theory developed.