Dear Doctor: I had a yeast infection, and even though I did everything the doctor told me to, it has come back twice. What causes a yeast infection? How do I keep from getting another one?

Dear Reader: About three-quarters of all women are regrettably familiar with the unique torment of a vaginal yeast infection. Also known as vaginal candidiasis, it’s a condition that occurs due to an overgrowth of a species of Candida fungus, most often Candida albicans. The result is irritation, discharge, odor and a nonstop, maddening itch.

An overgrowth of Candida can occur when something disrupts the pH balance within the vagina. This prevents certain bacteria naturally present in the vagina, including Lactobacillus, from successfully keeping the fungal growth in check.

Despite the discomfort, a yeast infection isn’t considered to be serious. Over-the-counter topical treatments like Monistat, which are used from one to three days, are usually quite effective. (Please note that many of these topicals are oil-based, which can weaken a latex condom and a diaphragm.) In more serious or persistent cases, oral fluconazole, a prescription medication, may be required.

When over-the-counter treatments don’t cure the infection, or when symptoms recur within eight weeks, it’s important to seek a medical opinion. Several other conditions, including a vaginal infection known as bacterial vaginosis, and trichomoniasis, a common sexually transmitted disease, may have similar symptoms to a yeast infection, but require different treatments.

Causes of a yeast infection include the use of oral antibiotics, which affect the balance of bacterial colonies throughout the body. Spermicides can adversely affect the vaginal environment, as can the prolonged use of tampons. Glycerin, which is present in many lubricants, can be a contributing factor in yeast infections. The mouths of up to half of adults contain Candida albicans, which has led researchers at the University of Michigan, Ann Arbor to link oral sex to an increased risk of developing a yeast infection. The hormonal changes that occur during pregnancy or menopause, a weakened immune system and high blood sugar can also be contributing factors.

To prevent yeast infections, remember that fungi thrive on warmth and moisture. Wear breathable underwear, and avoid prolonged time in a damp bathing suit, sweaty gym clothes or tight pants. If you use panty liners, change them frequently. Dry off thoroughly after bathing; don’t use douches or sprays, which adversely alter the chemical environment of the vagina; and steer clear of scented tampons, pads or other feminine products.

Unfortunately, about half of women will go on to have a second yeast infection, and an unlucky 5% are faced with recurrent vaginal candidiasis, which is four or more yeast infections per year. To reiterate: Whenever a yeast infection persists despite treatment, or when one recurs within a few weeks, contact your health care provider.

(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: After a mammogram, I was told I have dense breast tissue and that instead of a regular mammogram, I need to start getting a 3D mammogram. Why would that be? What exactly is dense breast tissue, anyway?

Dear Reader: A woman’s breasts are made up of several different types of tissues. There are glandular tissues, which produce, store and deliver milk; connective tissues, which support the breast and help give it shape; and fatty tissues, which make up the rest of the breast. Fatty tissues help protect and support the glandular tissues, and they play a role in breast size. Breasts have no muscles, but they do also contain blood vessels, lymph vessels and lymph nodes.

Dense breast tissue is indicated when your mammogram reveals a high proportion of connective and glandular tissue, and a lower amount of fatty tissue. Up to half of all women are considered to have dense breast tissue. Unlike fatty tissue, which appears as dark and transparent on a mammogram, dense tissue reads as solid white. So do the tumors that mammograms are used to identify. As a result, the presence of a high proportion of dense breast tissue can make it easier for cancers and other potential problems to go undetected.

Like regular mammograms, the 3D mammogram is an image of the breast obtained using low-dose X-rays. The procedure for the 3D mammogram is also the same. You stand in front of the imaging machine, and your breast is compressed between two plates (yes, ouch) to flatten the tissue and to prevent movement while the X-ray is being taken. Both types of mammogram are used to identify early signs of breast cancer.

The 3D mammogram is different because, as the name suggests, it creates a multi-dimensional picture of the breast. This is achieved by capturing many images of the breast tissue from different angles, which generates thin cross sections of breast tissue. These combine to create a clearer and more detailed picture of the breast. The 3D mammogram, also known as breast tomosynthesis, takes a bit longer to complete than the standard mammogram. It emits a similar amount of low-dose radiation to the standard mammogram, but since it’s often used in addition to a standard mammogram, the total dose of radiation is higher.

For some women with dense breast tissue, even a 3D mammogram may not offer adequate diagnostic information. In these cases, the use of magnetic resonance imaging (MRI) or an ultrasound may be required. Other types of imaging tests are also being studied. These include an optical imaging test, which analyzes the movement of light through breast tissue, and molecular breast imaging, or MBI, which uses a radioactive tracer to identify the presence of cancer cells. Electrical impedance imaging distinguishes normal tissue from cancer cells through the use of a very small electrical current. None of these tests has yet been approved for breast cancer screening.

(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’ve heard that the keto or paleo diets are good for weight loss and controlling blood sugar. Now I’m reading that a new study says they’re bad for your heart. What’s actually true?

Dear Reader: You’re referring to the ketogenic diet, also known as keto, and the Paleolithic diet, or paleo. Both are low-carb approaches to eating that are quite popular right now.

The paleo diet, also known as the caveman diet, draws from the foods that were presumably available to our Paleolithic ancestors. Since the Paleolithic era lasted 2.5 million years and ended about 12,000 years ago, the specifics of those diets are basically guesswork. But the core tenet of the paleo diet is that if a caveman didn’t eat it, neither should we. The result is a low-carb diet heavy on meat, with a limited amount of high-fiber fruit and vegetables. Since the Paleolithic era predates agriculture, things like grains, beans, cereals, legumes and dairy products, each of which require farming, don’t make it onto the plate.

The keto diet is even more restrictive. Its roots go back to the 1920s, when researchers found that a diet containing a very low proportion of carbohydrates and a very high proportion of fat appeared to reduce the number or severity of seizures in people with epilepsy. The goal of the diet is to put the body into a metabolic state known as ketosis. In ketosis, the body has depleted its stores of carbohydrates, its first go-to for energy, and instead begins to burn fat. This results in the creation of metabolic byproducts, including certain fatty acids, which are believed to ease the symptoms of epilepsy. Another side effect of ketosis is steady weight loss. The keto diet is heavy on protein and fat, including meat, fish, sausages, bacon, oils, butter, cheeses, nuts and seeds. The diet allows high-fiber carbohydrates, but in limited amounts.

It’s true that studies have associated both the paleo and keto diets with improved blood glucose control. They each also result in better appetite control and speedy weight loss. However, we don’t yet have enough reliable data about whether these diets promote long-term weight loss, or what health risks may be attached over time. The study you mentioned in your letter found a sharp increase in a certain blood biomarker linked to heart disease risk in participants who followed the paleo diet. At the same time, they had an increase in high-density lipoprotein, or HDL, the so-called “good cholesterol.”

Another study tied high-meat, low-carb diets to a drop in the diversity of the gut microbiome. An increase in the number of harmful bacteria was also observed. A separate analysis of a number of existing studies also found links between meat-heavy diets and an increase in cardiovascular risk. Bottom line -- we’re still sorting things out.

We understand the allure of the keto and paleo diets, but our advice continues to be a focus on whole rather than processed foods, lean proteins, a wide variety of fresh fruits and vegetables, whole grains, nuts and legumes, and healthful oils. Good for the heart, good for the gut and good for the planet.

(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.)