What is the single precaution that is known to prevent transmission of pathogens from one person to another?

The single most effective precaution to prevent the transmission of pathogens from one person to another is proper hand hygiene, specifically regular handwashing with soap and water. This practice effectively removes dirt, viruses, and bacteria from the hands, significantly reducing the risk of infection. When soap and water aren't available, using hand sanitizer with at least 60% alcohol can also help prevent the spread of pathogens.