The biggest worry is the spread of the virus itself, which on Friday hit sailors on a second aircraft carrier as the Pentagon reported military coronavirus cases among troops topped 300 — raising new questions about whether some of the military's frontline units might not be fully prepared to respond to an attack or could be sidelined altogether.

“We’ll be able to recoup if it's three months, four months, five months,” Rep. Elissa Slotkin (D-Mich.), a former Pentagon official and member of the House Armed Services Committee's readiness panel, said in an interview. “What we can’t allow to happen is a significant spread in our active-duty forces, either here or abroad.

"That would be a real problem if you had units that weren’t able to perform and show up and deploy," she added. "Then we start to get into a different conversation. I think the secretary [of Defense] is trying to stave that off because he knows that would be sort of hollowing out of his force.”

While the confirmed cases of the virus in the ranks are only in the hundreds, the numbers are expected to keep climbing, despite the countermeasures being taken to limit the spread.

An alarming situation is unfolding aboard the USS Theodore Roosevelt, an aircraft carrier that was deployed to the Pacific and is now pierside in Guam and where two dozen sailors have tested positive. None have been hospitalized or are seriously ill but the Navy is working to test all 5,000 sailors aboard the warship — a painstaking process that could take weeks. And then on Friday, Fox News reported that sailors had tested positive for coronavirus on a second carrier, the USS Ronald Reagan, which is stationed in Japan.

"We're taking this day by day,” Chief of Naval Operations Adm. Mike Gilday said in a statement Thursday regarding the Roosevelt. "Our top two priorities are taking care of our people and maintaining mission readiness. Both of those go hand-in-glove. We are confident that our aggressive response will keep USS Theodore Roosevelt able to respond to any crisis in the region."

But Defense Department leaders have raised increasing concerns throughout the week about how long it might be before they won't be able to make that same promise.

"If this pandemic continues at the scale and scope of what some are predicting, over time you could start seeing an impact on readiness," Defense Secretary Mark Esper told reporters on Tuesday.

On Thursday Esper said that the military would no longer reveal as many particulars about where the coronavirus is infecting the ranks to avoid tipping off adversaries.

“What we want to do is give you aggregated numbers. But we’re not going to disaggregate numbers because it could reveal information about where we may be affected at a higher rate than maybe some other places," Esper told Reuters. “I’m not going to get into a habit where we start providing numbers across all the commands and we come to a point six, seven weeks from now where we have some concerns in some locations and reveal information that could put people at risk."

Army Chief of Staff Gen. James McConville expressed confidence on Thursday that the service has not suffered any major setbacks so far.

"It's only been a couple of days, so to speak,” he said when asked about the scaling back or cancellation of a host of training exercises this month, including a freeze at the Army's three main field training centers in California, Louisiana and Germany.

But he said the Army needs to start planning for that to change.

“I mean, ideally, we want to keep people back to doing the missions that they need to do,” he said. “They're still able to train at the lower levels, exercising the proper procedures to limit exposure. But, you know, we certainly are going to be doing assessments as we move forward over the next couple weeks.”

Meanwhile, the Army issued a new order for personnel on Thursday calling for more aggressive actions to prevent the spread of the virus, warning that “mitigation measures taken by the Army to blunt the spread of COVID-19 have proven insufficient."

Air Force Gen. Arnold Bunch, in a public message to his forces at Air Force Materiel Command on Thursday, also said "while we continue performing our critical missions due to the resourcefulness, ingenuity and flexibility of our people, there will likely be future mission impact.

"We will continue to monitor the situation and take appropriate actions to minimize these impacts to ensure the readiness of our Air Force to answer the nation’s call," he added.

The virus is placing a host of stresses on the military’s day-to-day operations. And like the rest of the country, the creeping threat of the virus has reached numerous pockets of the military.

It began with preemptive steps such as cutting short a large-scale military exercise with NATO countries in Europe and banning on all domestic travel not deemed mission-critical. This week, the Pentagon took the unprecedented step of halting nearly all overseas military deployments for 60 days, affecting some 90,000 troops.

Over the weekend the Army announced it has cut the number of recruits in basic training in half after an initial six soldiers got sick with the virus.

Some specialists on readiness are particularly worried about the implications for boot camp for fresh recruits, given how much the military relies on new entrants.

“If the services shut down boot camp, that stops the flow of personnel into the military," said Mark Cancian, a retired Marine colonel who is now a senior adviser at the Center for Strategic and International Studies. "The military loses about 25 percent of its strength every year. That means it loses two percent of its strength every month."

He said that depending on how long the pandemic lasts, “that could really put a dent in military capabilities.” Canceling or postponing war games or field exercises is one thing, be added, but "if people don’t go in basic training, there’s nothing.

“The military has to be looking at how to cope with its training establishment," he added. "I would expect something pretty soon, frankly. That could be a shutdown. They could redesign the training, scale it back, pace people out. That gets at very fundamental aspects of military capability."

The need for the military to hunker down and take extra precautions to minimize the outbreak could have positive effects on military readiness, said Michael O'Hanlon, a military specialist at the Brookings Institution.

He pointed out that military preparedness is supported by many elements, not just day-to-day preparations and training. It also rests on mental health and family well-being.

"So you take advantage of this kind of a moment to rest," O'Hanlon said. "Most parts of our military can probably use a month or two of rest."

But ultimately, the military may have little choice but to accept a lower level of readiness and incur more risk, at least temporarily, he said.

"Maybe you just accept that a certain unit could have a higher likelihood of disease," O'Hanlon said. "But unless you really think you're going to need to send the 3rd Infantry Division over to Korea, you actually let that unit go to a little bit lower level of readiness."

But "we're going to have to have a strategy for how to get the military to start doing higher level training by the fall," added. "And I don't know if it's going to be people wearing face masks. I don't know if it's going to be that we give the military the first vaccines. I don't know if it's just that we rotate, so certain units are ready or have a higher number of people who are sick, and other units, we keep them healthy and don't train quite as hard at the same time."

Some like Slotkin worry about what those possibilities mean over the longer term, especially if potential adversaries think they can somehow take advantage.

“My constituents ask me, 'Are we at greater risk of attack right now? Is our military going to be able to handle that if one of our adversaries decides to take this opportunity to kind of punch us in the gut?'" she said. "While I still think the military is very capable of chewing gum and walking at the same time, the secretary [of Defense] has to be thinking about what it could mean if he had outbreaks in his active-duty forces."

Cancian, the retired Marine colonel, said it is paramount that the U.S. keep signaling its forces are ready so that potential enemies — even if they are struggling to control the virus, too — don't misjudge the situation.

“Adversaries may not view things the way we do," he said.