Hello, dear readers! We hope that you're settling into 2018. (It still sounds odd to say it!) We love that you're keeping our inbox full, and, as ever, we remain grateful for your interest and engagement in our columns.

Quite a few of your letters delve deeper into topics we have tackled and are slated to become the subjects of follow-up columns. To those who have written to thank us, we say thank you. And now, onward, as we address a few of your questions, thoughts and concerns.

-- A recent column about headaches and their causes prompted a reader from Madison, Wisconsin, to write in with an addition to the list:

"There is one more possibility that is often ignored," she wrote. "My son had headaches on and off between the ages of 12 and 17. At age 17, he had his wisdom teeth out, and the headaches were gone. People with headaches that start in their teens may want to check with a dentist to see if their wisdom teeth are impacted."

While not nearly as common as the causes discussed in the column, impacted wisdom teeth can trigger painful and repeated headaches in some people. These teeth, which are the third molars at the back of the mouth, often lack adequate room to emerge. Whether due to infection, pressure or misalignment of the jaw, the result can be recurring headaches. If the onset (or increase) in headaches coincides with the typical age range for wisdom tooth eruption -- 17 to 24 -- it's a good idea to check with your dentist.

-- The dental angle on headaches got a thumbs-up from another reader, who says she was initially a skeptic:

"The 'dental problem' was a cause that I didn't buy into," she wrote. "My dentist told me I was grinding my teeth, and though he made me a tooth guard, I didn't wear it for years. I finally gave it a try because my headaches persisted and pain meds were hurting my stomach. No more daily headaches!"

-- We loved this story from a reader from Naples, Florida, in response to our column about the importance of the annual physical exam.

"A friend of mine, who was feeling fine, went to her annual physical," she wrote. "Routine blood tests revealed that she had leukemia. She immediately sought treatment and today, 20 years later, she's fine. I can't think of a more powerful argument in favor of having an annual physical."

Needless to say, we heartily agree that an annual physical exam can be a lifesaver. Whether it's identifying "silent" problems like hypertension or the early stages of serious illness, or enlisting your primary care physician as your partner in long-term health and wellness goals, a yearly physical is a powerful -- and empowering -- tool.

-- Finally, we just learned of an interesting advance in cochlear implants, which we addressed in a column recently. New programming software that was just approved by the FDA will allow audiologists to evaluate and adjust a patient's implants from afar via remote programming software.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: Because I have fair skin and live in an area with a lot of sun, I try to do my outdoor activities early in the day (before 9 a.m.) or late in the afternoon (after 4 p.m.). Do I still need to wear sunscreen?

Dear Reader: In a word, yes. If your goal is to protect your skin from sun damage, which includes sunburn, wrinkles and a range of cancers, our advice is to wear sunscreen during the daylight hours. In fact, research now shows that even a light tan is a sign of sun damage.

This dawn-to-dusk timing for sunscreen application may seem counterintuitive; after all, sunlight looks and feels markedly weaker during the early morning and late afternoon than at midday. While it's true that the sun's rays are strongest between 10 a.m. and 4 p.m., the fact is that from the moment it appears above the horizon to the instant it vanishes from sight, our closest star is sending a full dose of skin-damaging ultraviolet radiation our way.

How can that be?

Sun damage is caused by two types of radiation -- ultraviolet A and B, better known as UVA and UVB. These are part of a broad spectrum of light, which includes the light we can see. But because ultraviolet rays are shorter than rays of visible light, they can't be seen with the naked eye. However, what you can't see definitely can hurt you.

The so-called "sunburn ray" is UVB. It has a slightly shorter wavelength than UVA, and is not as concentrated in the early morning and late afternoon. The highest amounts of UVB radiation reach us between 10 a.m. and 4 p.m. in the months of April through October. And while UVB accounts for only 5 to 10 percent of all UV radiation that reaches us, it plays a key part in the onset of skin cancer.

By contrast, UVA maintains the same intensity all day, which makes dawn-to-dusk sunscreen use important. UVA is responsible for signs of aging, like wrinkles and dark spots. It also damages certain cells in the basal layer of the epidermis, which is where most skin cancers form. In the past, UVA was absolved of a cancer connection. Now, however, researchers believe that it plays a role in cellular changes that lead to cancer.

The other thing to know is that UVA can penetrate clouds and glass. When you're riding in a car, sitting near a window or spending time outdoors in poor weather, you're still in the path of UVA rays. Reflective surfaces, such as water, snow, ice and glass, refract up to 80 percent of the UVA and UVB rays that hit them. That means you're getting close to a double dose of the sun's harmful rays. And don't forget that the sun's rays become more potent at higher altitudes.

Our final word of advice is to be sure to use sunscreen rated for both UVA and UVB radiation. It will be clearly marked on the label. Use it generously and reapply according to product instructions. Your skin will thank you.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: Every now and then I read about a death related to "flesh-eating bacteria" that was contracted in water of some sort. The most recent one was in the floodwaters of Hurricane Harvey, near Houston. How does this happen? Is there anything we can do to stay safe?

Dear Reader: When we talk about flesh-eating bacteria, we're really talking about necrotizing fasciitis, a serious skin infection that can be caused by one of several different types of bacteria. The bacterium enters the body through a break in the skin -- anything from an insect bite to a scrape, scratch or open wound -- and then rapidly spreads through the body's soft tissue.

Contrary to their name, these bacteria don't actually eat the flesh. However, they do kill it. The word "necrotizing" means "causing the death of tissues." The word "fasciitis" refers to the fascia, which is the connective tissue that surrounds muscles, fat, blood vessels and nerves.

As for the bacteria themselves, group A strep is the most common cause. Other possibilities include E. coli, staph (aka Staphylococcus aureus), Klebsiella, Clostridium and Aeromonas hydrophila. In the case of Harvey's floodwaters, which included seawater, the marine bacterium Vibrio is also a possible culprit.

Although several cases of necrotizing fasciitis came to light due to contact with the contaminated floodwaters of Hurricane Harvey, the condition is rare. About 700 cases are diagnosed per year in the United States. In fact, many people exposed to the bacteria that cause it are not affected. According to the Centers for Disease Control and Prevention, those who do contract it often have weakened immune systems due to conditions like diabetes, cancer, chronic lung, heart or kidney disease, or autoimmune diseases like lupus.

There is no surefire way to prevent necrotizing fasciitis. That means the best defense is using good wound care techniques. For example:

-- Never delay first aid for a wound, even if it's minor or not infected. A blister, scrape or any break in the skin should be cleaned, disinfected and covered.

-- If you do have an open wound, or one that is draining, always keep it covered with clean and dry bandages until it is healed.

-- If you have an open wound or a skin infection, steer clear of enclosed systems like hot tubs, swimming pools or whirlpools. You should also avoid contact with natural bodies of water, like rivers, lakes, ponds and oceans.

Vigilance is key. Necrotizing fasciitis moves swiftly. Unlike a typical infection, which develops over the course of days, the symptoms of necrotizing fasciitis begin within hours.

Many patients report experiencing pain that is more severe than expected for the size of their wound. Ulcers, blisters or black spots may appear. The skin may feel quite warm, with areas of swelling that have a reddish or purple cast. Later, flulike symptoms including chills, fever, extreme fatigue and vomiting may appear.

Successful treatment depends on the earliest-possible intervention. Even then, broad-spectrum antibiotics and surgical debridement may not stop the infection. In some cases, amputation becomes the only option to save the patient's life.

If after performing first aid you are concerned about a wound, or simply feel better being hypervigilant, use a pen to outline the borders of redness when any infection appears. If the infection grows beyond those borders in the course of hours, seek out medical help immediately.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)