Dear Doctor: Our city has asked us to wear a face mask when we leave the house. Needless to say, my wife and I have never done this before. Does our 5-year-old son need a mask, too? Any guidance or how-tos would be greatly appreciated.

Dear Reader: Your local government is following the updated recommendation from the Centers for Disease Control and Prevention, which urges people to wear a cloth covering over their nose and mouth whenever they go out in public. These face coverings don’t serve as protection from becoming infected with the virus. Rather, they are a physical barrier that may prevent someone who is infected from spreading it. This is particularly valuable in cases where someone is infected but has no symptoms.

This is very important, so imagine our voices as stern and serious when you reach the uppercase words in the next sentence. A face mask should only be used IN ADDITION TO and never INSTEAD OF the other important steps we’re all taking right now. These include social distancing, frequent and thorough hand-washing, not touching your face, and cleaning high-touch surfaces such as door handles, light switches, phones, eyeglasses and sunglasses. (And don’t forget about the handles of your car doors and your steering wheel.)

Face coverings should be snug but comfortable, fit against the sides of the face and beneath the chin, and allow for unrestricted breathing. They can be secured by ear loops or ties and should include multiple layers of fabric. Make sure the masks you choose can be washed and machine dried without damaging the mask or changing its shape.

When putting on a mask, start with clean hands. Always hold it by the strings or straps, not by the mask itself. Place it over your nose and mouth and adjust the straps so that you have a snug but comfortable fit. If your mask has a bendable metal clip over the bridge of the nose, gently adjust it to the contour of your face. If you wear glasses, put them on after the mask. It’s important that any facial covering you use, even if it’s just a bandanna, completely covers both your nose and mouth, and also the bottom of your chin. It’s helpful to practice at home to get optimal fit and comfort.

Before you remove your mask, clean your hands. Again, you’ll be handling it only by the ties or straps. If it’s disposable, throw it away. If it’s reusable, wash it with soap in hot water and machine dry thoroughly on medium or high heat. And please, never use medical-grade N95 masks. These are in short supply and are desperately needed to keep frontline workers safe.

Yes, your son should wear a mask in public as well. The CDC recommends everyone 2 years of age and older wear a mask that covers their nose and mouth. Due to the risk of suffocation, face coverings should not be used on anyone younger than 2. We hope this helps, and that you and your family stay vigilant, safe and healthy.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Hello again, dear readers, and thank you for joining us for this month’s letters column. It’s a difficult and challenging time, and we’re more grateful than ever for the community that has sprung up in response to Ask the Doctors.

-- A reader from Ohio, Illinois, who is her dad’s caregiver, asked for guidance on expiration dates on food items. “He buys more food than he needs, and it stays in the refrigerator well past the ‘sell-by’ date,” she wrote. “I’ve brought it up several times, but we just argue because he thinks it’s OK.”

You’re correct that it’s important to be vigilant about food freshness and safety. Older adults are at increased risk of hospitalization, and even death, due to food-borne illness. You mentioned your father’s handling of prepared foods, like baked chicken, as well as staples like eggs and bacon. It’s helpful to know that the sell-by dates on packaged foods refer to food quality rather than food safety. Eggs that remain refrigerated are safe to use for up to three weeks beyond the sell-by date. A package of bacon can be kept refrigerated for up to a week beyond the sell-by date. With prepared foods, however, the window becomes much smaller. When refrigerated properly, cooked chicken will last for three to four days. After that, it’s not safe and should be thrown away.

-- A reader from Omaha, Nebraska, wondered about health benefits attributed to a few teaspoonfuls of apple cider vinegar per day. It’s an age-old folk remedy, and while many of the claims are far-fetched -- no, you can’t replace blood pressure meds with apple cider vinegar -- there is some evidence that it can be helpful. For instance, studies have shown that apple cider vinegar may play a modest role in control of blood sugar. It may also aid in weight loss by suppressing appetite. However, apple cider vinegar is an acid, which can erode tooth enamel. Be sure to always rinse your mouth thoroughly after having any.

-- A reader from Dallas, Pennsylvania, sent us an ingredient label from his favorite food, a prepared meal of Salisbury steak with macaroni and cheese. He asks if it contains dangerous ingredients. Although the ingredients themselves are benign, the nutritional information has some bad news. One serving contains 1,110 milligrams of sodium, which is almost half the recommended daily value. The saturated fat content is also a bit high, at 30%. Rather than a daily meal, it would be wise to make this a weekly treat. Instead, add some lean meat, fresh vegetables, whole grains and healthy oils to your diet. Your body -- and your family doctor -- will thank you.

-- After reading a column about improving indoor air quality during a wildfire, a reader from Tulsa, Oklahoma, was baffled by the advice to refrain from dusting or vacuuming. “How do you keep the house clean?” he wrote. “Isn’t it impossible?” Those tasks kick up significant particulates, so it’s wise to defer them when air quality is bad, as during a wildfire. Once the outdoor air has cleared, it’s safe to dust and vacuum.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: I heard on the news about a coronavirus medicine made with blood from people who were sick and got better. But my husband says it’s actually a blood test to see if someone ever had the virus. Who’s right?

Dear Reader: You and your husband are both correct. Two different uses for antibodies have recently been greenlit by the U.S. Food and Drug Administration.

You’re referring to an experimental treatment that was approved on March 24 for very limited use. Your husband is talking about a quick and simple blood test approved by the FDA on April 2. The test, which is already in use in other parts of the world, reveals whether or not someone has been infected with the novel coronavirus. The treatment and the test each rely on antibodies, which are blood proteins that the immune system produces during and after its fight against a foreign invader, such as a virus or bacterium.

Let’s start with the therapy you’re asking about. It’s based on the idea that, when introduced to antibodies used to fight a specific pathogen, a sick person’s immune system will produce those very antibodies. This type of treatment has been in use since the 1890s for diseases such as polio, SARS, Ebola and the H1N1 flu. The idea of using it to treat COVID-19 arises from a very small pilot study -- just six men and four women -- recently conducted in three hospitals in China.

Each of the patients, who were gravely ill with COVID-19, received a single dose of antibodies gathered from the blood of patients who had recovered from the illness. According to the published research, the treatment resulted in improved oxygen levels and a shorter duration of symptoms in all 10 patients. Most improved within one to three days after receiving the antibodies. The patients also cleared the virus from their bodies more quickly than those who did not receive the treatment. This sounds promising, but it’s important to note that the FDA approval limits treatment to patients who are so ill that they have no other options, and to clinical trials.

The antibody test that your husband is referring to is much closer to a wide release. It uses a pinprick of blood and delivers results in about 15 minutes. Unlike the diagnostic test for COVID-19, which identifies genetic material from the virus itself, this test looks for the antibodies that will be present only if the body has already encountered and fought off the virus. This is valuable information on two fronts. First, widespread use of the antibody test will give scientists a clearer picture of the spread and reach of coronavirus infection in the U.S. Just as important, the test hints at a potential path forward after the height of the epidemic has passed. People who have been exposed to the virus may have some degree of immunity, which would allow them to safely rejoin normal daily life. The challenge here is that, at this time, how much immunity is granted by prior infection, and how long that immunity may last, is not yet known.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)