As they were walking into work Tuesday, employees at Amazon's fulfillment center in Moreno Valley, California, learned that someone in their facility had just tested positive for COVID-19, the disease caused by the coronavirus.

"I first heard about it on Facebook," an employee at the fulfillment center, who asked to remain anonymous for fear of retaliation, said via text message Wednesday. "Then I confirmed it when I went up to the Amazon parking lot across the street when lots of people were leaving for home frightened because they didn't get notified through email."

"No one knew about it until going into work," she said.

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That was the first case of COVID-19 that Amazon has confirmed in one of its 14 fulfillment centers in the Inland Empire region of California, just east of Los Angeles. The cluster is Amazon's largest concentration of warehouses and fulfillment centers in the world, employing about 18,000 people, according to a study by the Economic Roundtable, a Los Angeles-based nonprofit organization.

The Moreno Valley center is the latest warehouse where Amazon workers have tested positive. Amazon has had positive cases of COVID-19 in 10 of its fulfillment centers in various parts of the U.S., CNBC reported Thursday. It temporarily closed a warehouse in the New York City borough of Queens last week after a worker tested positive. Amazon has also closed a facility in Shepherdsville, Kentucky.

Amazon hasn't closed the Moreno Valley center, according to two employees who work there and asked to remain anonymous because they were not authorized to speak to the media.

The growing number of Amazon centers where employees have tested positive has sparked concern from warehouse workers, as well as from Amazon's technology workers, who are agitating within the company for more to be done to protect the health and livelihoods of warehouse workers.

Amazon hasn't published details about the closures.

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Few locations are as crucial for Amazon as the Inland Empire. Amazon employs between 1,000 and 5,000 people at the Moreno Valley fulfillment center alone, according to state figures.

"It would crimp the retail supply chain to folks in the greater Los Angeles region to have this warehouse and other warehouses stop operating," said Daniel Fleming, president of the Economic Roundtable. "There's a lot of reliance on it."

The situation underscores the tenuous but crucial role Amazon plays in the U.S. supply chain.The coronavirus outbreak has pushed more people to order food and household staples online, increasing the strain on Amazon's system. The company has said that it plans to hire 100,000 workers to handle the demand and that it has limited its warehouse space for household staples and medical supplies.

The two Moreno Valley employees said that they're not being offered paid time off if they're afraid to work because of a threat of infection but that they are being allowed to go home without pay and not lose their jobs. It's a decision, they said, between making the money they need to support their families and keeping their loved ones safe from the fast-spreading virus.

"It's not fair that we have to suffer," a second worker said. "We have bills and family to feed. Our worry is going to work and coming back to our family and not knowing if we have the virus or not."

"Some of our family members could have weak immune systems due to being very old and very young," she said. "We truly need to shut the building down. It's not fair for us."

In a blog post Tuesday, Amazon said it was consulting with health authorities and medical experts on how to handle building closures for cleaning when employees test positive.

"Our process evaluates where the employee was in the building, for how long, how much time has passed since they were onsite, and who they interacted with, among other items, in determining whether we need to close," according to the company statement. "We also ask anyone at the site who was in close contact with the diagnosed individual to stay home with pay for 14 days in self-quarantine."

Some workers in the Moreno Valley fulfillment center learned of the positive case at the end of their shifts when they were told by their managers, the two employees said.

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Not all Amazon workers are being forced to decide between getting paychecks and not making their families sick. Those who build and maintain the technical infrastructure of the e-commerce and cloud giant are able to work from home.

On Wednesday, a group of Amazon tech workers who have previously called for changes within the company circulated a letter to more than 600 colleagues on an internal mailing list about how they could support warehouse workers.

"Even as office workers are asked to work from home, Amazon's measures to protect FC (fulfillment center) and DC (data center) workers, as well as shop floor workers at Whole Foods, have ranged from inadequate to openly negligent," said the letter, a copy of which was obtained by NBC News. The letter encouraged colleagues to donate to a GoFundMe page set up to support fulfillment center employees who have contracted COVID-19.

A software development engineer at Amazon, who asked to remain anonymous because of rules against speaking to the media, said: "Warehouse workers are our co-workers. We are responsible for how they are impacted by the tech that we are creating and profiting off of."

The spread of the virus inside warehouses has fueled concern on social media from people worried about the possible spread of the virus through the packages that they get at their homes. Some have discussed cleaning their packages after receiving them. Experts, though, have said the risk of transmission from a delivered package is low.

Researchers reported this month in the New England Journal of Medicine that, in tests for the coronavirus on a variety of surfaces, no viable virus was measured on cardboard after 24 hours. But in a warehouse, a single package can be touched by over 15 employees during a 24-hour period, according to the warehouse workers who spoke to NBC News.

"We are one of the largest and best-positioned companies on Earth at the start of this crisis," said a second employee, who asked to remain anonymous because he isn't permitted to speak to the media. He's working at home, as is everyone else on his team.

"It's infuriating to watch us fail to be leaders when it comes to supporting our warehouse staff and delivery drivers."