Unlike bacteria, viruses are not living organisms. They cannot live autonomously because they have neither their own cell nor their own metabolism and cannot produce food for energy. Viruses need another cell (host cells are animal, plant or human cells) in order to live and reproduce. Only with the aid of this host cell can viruses multiply and spread. For the behaviour of viruses in the air, not only is carrier aerosols’ size decisive, but also the indoor climate, the air exchange rate and the way in which ventilation is carried out.

Bacteria are single-cell organisms with their own metabolism and genetic material. They feed themselves and reproduce via cell division. They make what they need for living within their cell. Bacteria are found everywhere, for example in the air, in water or in food. However, air isn’t bacteria’s natural habitat. They get into it somewhat accidentally through wind or spray, but also via excreting, sloughing off skin scales or by breathing, talking, coughing or sneezing. How long bacteria remain viable in the air depends on their tenacity (ability to survive). About one per cent of all bacteria cause human diseases when they enter the human body. In the process, they produce metabolic products that cause symptoms of illness.

Viruses, bacteria and germs can spread quickly in closed rooms where people are present. Viruses don’t move individually in the air, but rather are enclosed in larger droplets. When breathing, everyone emits tiny droplets (aerosol particles), which multiply rapidly in the air and may lead to an increased concentration of infectious particles. Each breath may contain 1,000 to 50,000 droplets. Since aerosols follow the air flow and float in the air, they can re-enter the body via inhalation. This makes disease transmission via aerosols particularly high. When coughing or sneezing, these droplets are ten times larger. If the supply of fresh air is limited, not only do germs and viruses increase, but also the risk of infection and contagion indoors.