Nearly 4,000 cases of the coronavirus have been confirmed in every state in the US — except one. Officials in West Virginia say it's only a matter of time that Covid-19 tests return with positive infections, while the governor has ordered schools to close while the state braces for the inevitable.

As of Monday, the West Virginia Department of Health and Human Resources reported no confirmed Covid-19 cases in the state after 84 tests were performed at its public health lab. Results from 80 of those tests were returned negative, and four tests are pending. Nevertheless, West Virginia has declared a state of emergency for its 55 counties.

At a press conference on Saturday, Governor Jim Justice said: "We know it's here. I mean, let's be real: It has to be here. We just haven't found it yet."

Residents' incidental isolation in the largely rural state has warded off the virus, for now. Health officials believe West Virginia's spread-out populations and little international air travel has reduced the risk of viral spread throughout communities, but state health officer Cathy Slemp said the lack of public coronavirus testing also has stunted the state's outreach.

The currently invisible threat could drastically impact the state's vulnerable population, among a handful of states where roughly half of adults are at a "higher risk of serious illness" if exposed to the coronavirus.

According to a recent report from the Kaiser Family Foundation, the virus could seriously threaten 735,000 people in West Virginia, or more than half of all adults in the state, Including 60 per cent of people age 60 and older. In the state-by-state review, calculating other health complications, West Virginia is the only state where more than half of adults are at a significantly higher risk of serious health issues.

Governor Justice said: "We're an older state, and the elderly is where this monster attacks."

The state also has more smokers than any other state and the highest mortality rate due to diabetes, as well as high rates of chronic respiratory disease, heart disease and cancer, which compound risk factors for a lethal Covid-19 infection.

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Governor Justice said he hopes the preemptive school closures — and preventing elderly school staff from potential exposure — are able to prevent overwhelming the state's hospital systems.

He said: "If this thing were to turn really ugly, are our hospitals able to handle a surge in the numbers of people that we could possibly have? God forbid, that would happen anywhere ... We've got a monster that's looming, but the monster's not here."

The state is confident it can "handle" an outbreak in the state because of its early preparedness, including warning against large gatherings, as per Centers for Disease Control and Prevention recommendations.

He said: "There's no way this monster is going to get us. But let's be smart."

Roughly 40 per cent of adults in the US — approximately 105.5m people — are at a higher risk of developing serious illness if infected with coronavirus, according to the Kaiser report.

Most of those people are older (72.4 percent, or 76.3m adults), while roughly 30m people ages 18-59 are at risk due to an underlying medical condition.

The number of confirmed cases in the US has climbed to nearly 4,000.

West Virginia's schools are closed through at least 27 March.

In a statement, Governor Justice said" "At the heart of everything we are doing right now is the protection of our children, making sure our schools are safe for our teachers and staff, and making every effort to protect all of the people of West Virginia."

The state has roughly 200,000 students from low-income households who rely on school meals throughout the week. The governor's office is working with county-level school boards to coordinate school buses to send meals to students at home.