The stinky stuff that dwells underneath the Michigan State University campus is no longer just simply waste.

It has become a public health tool.

Colleges across the U.S. that are grappling with the coronavirus are testing sewage for the presence of genetic material from the virus.

“It’s being used on campus as early warning and communication, so to let the students know that in their dorm there’s virus excretion, even if they don’t have cases,” said Joan Rose, the chair of water research at Michigan State.

Experts say it can be an inexpensive way to monitor people who may not even know they are sick yet.

“To be able to detect an asymptomatic or prior to clinical test positive individual, right, through the sewage surveillance program is important to avoid major events,” said Mahesh Lunani, the CEO of Aquasight LLC.

The Troy, Michigan-based company partnered with Oakland University to set up a laboratory at the school to test wastewater samples for the presence of the virus.

Researchers say the results can’t yet reliably show how many infected people live in a community.

But they can indicate if that number is rising or falling.

And that indication can arrive days before such trends show up by standard testing or hospitalizations.

“Really, it’s not much different than a blood test. When you go to the bathroom, you’re shedding virus. And you could be asymptomatic, but yet you continue to shed the virus,” said Candice Miller, the public works commissioner in Macomb County, Michigan, home to a number of population-rich Detroit suburbs.

Miller and other officials hope the lessons learned from the sewage testing efforts for the coronavirus can be applied to future health concerns.