This RT card provides basic information about infections, and the spread and prevention thereof. This issue is examined from the standpoint of a building's indoor spaces. What factors make up a hygienic indoor space, how can such spaces be improved, and why is it advisable to improve them? This instruction card is intended for all parties involved in building projects: building owners, designers, builders, contractors, and users. This instruction card is part of the Hygiene in indoor spaces instructional series. The other parts of the series are Spatial planning and Cleaning and maintenance. This RT card was made for the conditions of Finland.