To reduce the possibility of sewage backing into a home, homeowners need to seal areas where sewage can flow in during periods of excessive rains or flooding. Sewage not only can damage building components and carpeting, it also has high concentrations of bacteria, protozoans and other pathogens that can pose serious health risks. Water will seek the lowest level, so if the level of sewage or floodwater is higher than the drains in the home, such as those in the basement, a backup can occur.

Plugging Home Drains to Prevent Sewage Backup - 8-page publication

Planning Ahead: Plugging Household Drains -- 13-minute video