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.)

Dear Doctor: In the last few years I've noticed that after making an appointment to see a doctor, I'm often seen by a physician's assistant or nurse practitioner instead. What's the difference? When should I insist on being seen by a physician?

Dear Reader: You're correct -- nurse practitioners and physician's assistants are now playing larger roles in preventive health care for many Americans. While they can't replace physicians, what they can do is take on a significant range of duties in primary care. This frees up doctors to manage the big picture of a patient's health care and to spend more time with difficult, complex cases.

Let's start with some definitions.

The difference between a physician, a physician's assistant and a nurse practitioner is in their schooling. Each is a nationally certified and state-licensed medical professional who has successfully completed four years of college as an undergraduate.

Aspiring physicians go from college to four years of medical school, followed by an additional three to seven years of specialized training as residents. Many then undergo additional training in fellowships, which can last from one to three years.

For physician's assistants, or PAs, college is followed by three years of medical training in an accredited program. More than 2,000 hours are spent in clinical rotations that include family medicine, internal medicine, general surgery, emergency medicine, pediatrics, and obstetrics and gynecology. In order to enter practice, PAs must pass a national certifying exam and qualify for a state license. All PAs are required to work with a supervising physician.

A nurse practitioner, or NP, is a registered nurse whose advanced training allows her or him to perform an expanded range of duties. These include physical exams, diagnosing and treating chronic diseases like diabetes and arthritis, ordering and interpreting diagnostic tests, and prescribing medications. An NP can perform procedures like stitching a wound, setting a break and performing a skin biopsy. Their focus is preventive medicine, wellness and education. Depending on the state, NPs may work independently of a physician's supervision.

Both PAs and NPs have a role in surgery, but to what extent depends on laws and regulations, which vary greatly from state to state.

Like many of our colleagues, we see a role for PAs and NPs in preventive care, such as well-woman and well-baby exams, geriatric assessments, and in urgent care cases that are not complex or severe. Here at UCLA, many hematologists and oncologists work alongside an NP. The NPs help care for complex cancer patients, coordinating imaging, therapy and routine follow-up visits.

By handling certain parts of a medical visit, like collecting data, compiling a medical history and assessing general health, PAs and NPs free up physicians' time so they can focus on more complex issues.

This leads us to the second part of your question, which is when to ask to see a physician. We suggest that you make it a practice to ask whether a doctor is available during your appointment. That way, if an exam or test reveals anything troubling or abnormal, if you want a second opinion about something the PA or NP has said or if you have questions that you prefer to have answered by your doctor, then you know he or she is available to 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: The back of my neck hurts when I have my hair shampooed at the salon. This worries me because I've read that some people have strokes when they have this type of shampoo, due to the arteries in the neck. Is there truth to this?

Dear Reader: While it's true that some individuals have experienced strokes after spending time in a shampoo chair at a hair salon, it's actually quite rare. And don't worry -- there's more to it than simply leaning one's head back for a few moments.

An early take on the topic appeared in the Journal of the American Medical Association in 1993. Since then, additional research and case studies have been published, and occasional cases of women -- and a few men -- suffering strokes after a visit to the shampoo chair have made a splash in the headlines.

Most recently it's been in the news because a woman filed a lawsuit against a hair salon in San Diego, claiming a neck injury she suffered while getting a shampoo led to a stroke two weeks later. Specifically, she said an artery in her neck was damaged when she angled her head back and rested her neck against the hard surface of the sink. This reportedly caused bleeding, which led to a blood clot, which caused a stroke.

In media coverage, the injury goes by the name "beauty parlor stroke syndrome." In reality, though, what we're talking about is cervical artery dissection. That is, a rip or tear in one of the arteries in the neck, which supply blood to the brain. And despite what the injury's colorful nickname implies, it has multiple causes outside of the shampoo chair.

The injury begins with the anatomy of the neck, which includes arteries that run along the back of the vertebrae and deliver blood to the brain. In the people who experienced the so-called beauty parlor stroke, either one of two things appears to have happened: An artery was damaged by pressure from a vertebra, or pressure from the rim of the sink or the back of the chair squeezed the artery shut. In either scenario the result was the same -- a lack of blood flow to the brain, which is the definition of stroke.

This same injury has been seen in individuals undergoing prolonged dental work, receiving anesthesia, having chiropractic manipulations of the neck, and even painting a ceiling or doing yoga. In each case, hyperextension of the neck played a part.

If you're worried about your safety while getting a shampoo, talk to your hairdresser. Make sure the chair is adjusted properly for your height. Ask for additional towels for padding and support. Don't allow your neck to hyperextend. Or skip the backward approach entirely and ask to stand up and lean forward while your hair is washed.

Neck pain is not necessarily a sign that something is wrong. However, if you experience symptoms like loss of muscle tone in the face, weakness or paralysis on one side of the body, slurred speech, impaired vision or loss of coordination, you should definitely -- and immediately -- seek medical attention.

(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.)