Nightmare alert.

Washington health officials warned residents that heavy rain and floodwaters could sweep rats into the sewer systems and up into their ... well ... toilets!

The Seattle and King County public health department issued the warning in a Facebook post on Thursday.

"The heavy rain and floodwaters may sweep rodents into the sewer systems," the post said. "If a rat visits your toilet, take a deep breath and follow these tips," it added, along with infographics outlining what to do if a rat appears in your commode.

The infographics instruct residents to stay calm, close the toilet lid and flush. If the rat is still there, they should pour dish soap into the toilet and flush the toilet again until the rat is gone.

The warning comes after Washington state faced a deluge of historic flooding this month, which caused extensive damage of roads and other infrastructure.

This article was originally published on NBCNews.com