This study investigates the potential transmission of viruses through sanitary plumbing systems, focusing on a small-scale bathroom pilot test and numerical analysis in a hospital setting. The experiments revealed that under typical conditions, airflow and viruses could travel through piping due to room pressure differentials. However, when water seals inside traps were maintained, virus transmission did not occur in uninterrupted conditions. Conversely, loss of water seals led to induced airflow and virus transfer when the room's pressure differential exceeded 2.5 Pa, emphasizing the role of negative pressure in interconnected bathrooms. Numerical analysis indicated that isolating the infectious disease isolation facility's plumbing system from the general hospital's system prevented virus transfer through sewage pipes. The study concludes that fundamental preventive measures involve separating plumbing systems and ensuring adequate trap numbers to preserve water seals. These findings provide a basis for establishing installation and operation standards in sanitary plumbing systems to curb infectious disease spread, highlighting the critical importance of maintaining water seals to mitigate potential virus transmission through indoor air.