A quiet anxiety grips Palo Alto and Mountain View as residents face the possibility that the coronavirus will knock on their doors.

The Santa Clara County Public Health Department has identified 20 cases of COVID-19, the disease caused by the new virus, in the area. And as the number of cases climbs, residents are scouring grocery and drug stores to prepare to bunker down in their homes in case there's an outbreak or they find themselves infected.

Palo Alto resident Monica Hmelar has been searching several retail stores for hand sanitizer. On Monday, she stopped by the Costco store in Mountain View, but the warehouse ran out of everything she needed. One employee announced to customers that record numbers of people have been showing up as the store opens to grab basic necessities like toilet paper and water bottles. On Friday, 958 customers came through the store as it opened, the highest customer count in recent history, according to the employee.

"They told me to come back tomorrow at 9 a.m. when they'll have toilet paper," Hmelar, 52, said. "So I'm gonna be here at 9 a.m. to buy toilet paper."

None of Hmelar's family knows she's at Costco stockpiling for an outbreak — "They're probably thinking I'm crazy," she said — as the virus looms dangerously close to them.

Hmelar's youngest daughter is a senior at Palo Alto High School, where one student was sent home on Friday after the school district learned that the student's parent had been exposed to the virus, prompting the district to disinfect the campus. (The parent also has a child who attends JLS Middle School and who was also sent home on Friday as a precaution.)

And Hmelar's oldest daughter had recently traveled to Florence, Italy, before jetting off to London after receiving alarming news about the increasing spread and death tolls from the virus. On the same day the school district announced a parent was exposed to the virus, the Centers for Disease Control and Prevention raised Italy to a warning level 3, advising the public to "avoid all nonessential travels" to the country where the coronavirus has killed more than 50 people and resulted in about 2,000 confirmed cases.

"I'm kind of freaking out about the whole thing," she said. "And when I come to the stores it's not helping."

Retailers all over the Midpeninsula are rushing to restock shelves to meet the specific demands of anxious customers like Hmelar — such as hand sanitizers, masks and gloves — all of which have been flying off the shelves of local stores and online marketplaces. Many also have purchased freeze-dried goods and cough syrup.

"I ordered the masks," Hmelar said. "They're selling them on eBay for up to $1,000 dollars. I found some guy in South Carolina — a box of 10 for $100 dollars."

U.S. Surgeon General Jerome Adams issued a public plea over the weekend asking people to stop stockpiling masks. The Feb. 29 post on Twitter reads: "Seriously people - STOP BUYING MASKS! They are NOT effective in preventing general public from catching #Coronavirus, but if health care providers can't get them to care for sick patients, it puts them and our communities at risk!"

The World Health Organization has information on how and when to use masks here.

Though masks and hand sanitizers are increasingly hard to purchase, stores around Palo Alto don't exactly look like they've been hit by an apocalypse. Most of their shelves are fully stocked and continue to be refilled as more shipments arrive.

But inside places such as the Mountain View Costco, the neighboring REI, Palo Alto's Whole Foods as well as Town & Country Village's CVS Pharmacy and Trader Joe's, the unease of the community can be seen in the empty pockets of certain aisles that once stored rows of pasta sauces, canned tuna, toilet paper, freeze-dried mashed potatoes and zinc lozenges.

"It's just happening all in real time," said Michelle Kraus, a tech executive and political strategist who had just finished shopping at Whole Foods. "I don't know if I'm scared, but I'm uncomfortable. I'm the kind of person who needs information to decide what I can do."

To prepare for an outbreak, Kraus is stocking up on travel-sized hand sanitizers and zinc lozenges, an item that's curiously out of stock in some stores. The lozenges became popular after a pathologist's letter that instructed people to "stock up now" went viral.

The CDC does not list zinc lozenges as a protection against the coronavirus and there is no antiviral treatment available.

At Country Sun Natural Foods on California Avenue in Palo Alto, CEO Scott Otte also noticed his store was hit by an influx of concerned customers clearing out the inventory of pasta, rolled oats, disinfectant wipes and alcohol-free hand sanitizers. (The CDC only recommends the public use alcohol-based hand sanitizers if water and soap are not an immediate option.)

"We definitely got a run over the weekend," Otte said. "Our sales were about 20% higher than normal for a Sunday, and Sundays are already a busy day."

Otte chuckled with a little resignation as he thought about the prospect of closing his store — "a last ditch effort" that he would only consider if he's instructed to or if too many employees get sick.

"Sales have to be very, very low before it's not worth being open," he said. "There's rent and electricity to keep everything cold. Those are fixed costs regardless if we're open or not."

Abene Mendizabal, a Stanford University resident and mother of two, refuses to panic even as she works in the tourism industry, which has been directly affected by the virus through travel restrictions.

"I'm not doing anything because I already went through so many health issues in my life — I need to keep calm," she said while outside of Whole Foods after a routine shopping trip. "That's why I bought Champagne."

Stanford Law professor Michelle Mello, who also teaches health research and policy, doesn't see the need for people to purchase tubs of hand sanitizer, but believes the best measure is for people to carefully listen to their local public health department.

"We have excellent local leadership, and what they're saying is that we should be ready for life to be disrupted for a little while," she said. "That's not the same as panicking, but it's sensible for families to have a plan in place. If they're asked to stay in their homes for a couple of weeks to rule out an infection, will they be able to do that? In the same way we prepare for earthquakes, we should prepare for this foreseeable risk."

To prevent further spreading of the virus, the county's Public Health Department recommends taking the same precautionary measures as one would against any viral flu or infection: Wash your hands with soap; use alcohol-based hand sanitizer when hand-washing isn't available; cover your mouth with a tissue and properly dispose of it; stay home if you're sick; and start preparing your household in case a family member gets sick and needs to be isolated.

To find more updates from the city of Palo Alto on the coronavirus, visit cityofpaloalto.org.

Read our latest updates on local coronavirus cases here.

Find comprehensive coverage on the Midpeninsula's response to the new coronavirus by the Almanac, Mountain View Voice and Palo Alto Online here.