Dear Doctor: What's the difference between probiotics and prebiotics? I'm really interested in how gut bacteria impact health, but I'm not sure I always understand the terminology.

Dear Reader: We agree with you that the information emerging about gut bacteria is fascinating. And as with any area of research, the language used to discuss it can be confusing.

Probiotics are live microorganisms that are meant to have health benefits for the person who ingests them. They work by joining the many trillions of bacteria already living within our bodies, primarily in the gut. Collectively known as the human microbiome, these beneficial strains of bacteria harbor a diversity of genes that dwarfs that of the human genome.

Thanks to the rapid evolution of genetic sequence technology, researchers are learning new information about the human microbiome every day. Thus far, probiotics show promise in the areas of immune function and various digestive and bowel disorders. Probiotics have also been used to help preterm infants acquire a beneficial range of intestinal flora, which can prevent colonization by adverse bacteria.

You can ingest probiotics in food or in supplement form. When it comes to food-based probiotics, the common denominator is fermentation. Foods like yogurt, kefir, apple cider vinegar, pickles, kombucha, sauerkraut, kimchee, miso and certain soft cheeses are all sources of different types and varying amounts of probiotics.

The idea is that by ingesting these microorganisms, the portion of them that survive the acids of the stomach will take up residence in the gut. Once there, though, they need their own food source to grow and thrive. And that's where prebiotics come in.

Prebiotics are indigestible carbohydrates that act as food for probiotics. While some foods offer a greater percentage of prebiotic material than others, when you eat a diet high in a variety of vegetables, fruit and leafy greens, you are inevitably giving your gut bacteria plenty to eat.

You'll find a motherlode of prebiotics in foods like leeks, asparagus, Jerusalem artichokes, bananas, onions, garlic, artichokes, jicama and honey. Also beneficial is something known as "resistant starch," which is the indigestible portion of starch. It can't be broken down by the small intestine, so it moves to the colon, where it is fermented by the microbiota.

Whole cereal grains, many seeds and green bananas are good sources of resistant starch. So are cooked potatoes and rice, but only when they have cooled. Just as with probiotics, you can go the supplement route for prebiotics if you choose.

If you decide you'd like to add probiotic supplements to your diet, we suggest you check with your family doctor for guidance. Although regulated by the FDA, probiotic supplements are treated as a food and not a medication. That means that manufacturers don't have the burden of proof that their products will live up to the claims printed on the label. Your doctor can help you figure out which supplements to choose, and the best dose for you to start with.

(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: What can be done for normal pressure hydrocephalus? My dad was diagnosed with this condition six months ago after developing severe gait problems, urinary urgency and memory issues. He has a few other physical issues, but is otherwise healthy.

Dear Reader: I'm glad your father's doctor was able to pinpoint the source of his symptoms. Because normal pressure hydrocephalus (NPH) is a rare condition, with a gradual onset of symptoms, it can be difficult to diagnose. The annual incidence is between 1.19 and 3.4 per 100,000 people. This form of hydrocephalus, or "water on the brain," can occur after a head trauma, stroke or brain tumor, but in some cases, the cause is unknown. To understand the condition takes an understanding of the fluid that bathes the entire brain and that flows through its ventricles, or cavities.

This fluid, called cerebrospinal fluid, is produced by the lateral ventricles of the brain and is eventually reabsorbed into the body through veins within the brain. If the body has difficulty reabsorbing the fluid, the fluid increases within the ventricles, putting pressure on the brain itself. This pressure damages the nerves and nerve connections within the brain. Initial symptoms include difficulty walking, as if a person's feet are glued to the floor, and with the feet outwardly rotated. Patients also have difficulty turning and can lose stability and fall.

People with the condition also have concentration problems, slowed mental abilities and a lack of concern for their disability. Lastly, they have urinary urgency, which can lead to urinary incontinence.

Once you have a diagnosis, however, you can move to treatment, which consists of decreasing the level of cerebrospinal fluid within the skull.

This is done through a ventriculoperitoneal (VP) shunt. After drilling a small hole in the skull, a neurosurgeon places a catheter into the lateral ventricle within the brain. That catheter is attached to a valve behind the ear. From the valve, another catheter is guided down the neck and into either the chest or abdomen. When the pressure builds in the ventricle, the valve opens, draining the fluid from the brain into the chest or abdomen, depending where the second catheter ends. This fluid is then reabsorbed by the body.

The effectiveness of shunts varies widely, studies suggest. The greatest patient improvements seem to be in the ability to get up out of a chair and walk. Memory and reaction time also can improve after shunting, as can urinary symptoms. However, if memory deficits are to the point of moderate to severe dementia, then a shunt may not help that aspect.

One important thing to note is that the longer the patient has symptoms, the less likely a shunt will provide benefit. This is especially true if symptoms have been present for more than two years.

VP shunts can have many complications, including a risk of bleeding, brain infections, seizures, malfunction of the shunt or an over-drainage of cerebrospinal fluid. The rates of these complications are decreasing, but still can occur up to a third of the time.

Based on what you've told me, the cause of your father's hydrocephalus is unclear. Nonetheless, if the shunt is stable, patients like your father can have sustained benefit for many years.

(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: With cooler weather here in Minnesota, our kids, who love swimming, are switching from the lake to the pool at our local Y. Just how dangerous is it to swim in a public pool? I keep hearing there's a bug that chlorine doesn't kill.

Dear Reader: Any time you share a common space, whether with one other person or a crowd, you run the risk of being exposed to whatever germs, viruses, parasites, bacteria, molds or other environmental hitchhikers they may have brought along. Unsurprisingly, this applies to the public swimming pool, where diarrhea is the most common recreational water illness. And while chlorine significantly reduces the potential health risks posed by public swimming pools, it's not a guarantee.

When used in the proper amounts needed to maintain a consistent pH, chlorine eliminates all manner of nasty bugs. Chlorinated pool water kills E. coli in less than a minute. Norovirus and Hepatitis A perish after 15 minutes. Giardia lasts for only about 45 minutes in properly chlorinated water.

However, Cryptosporidium, a microscopic parasite that causes a diarrheal disease that can last two to three weeks, is different. Due to a protective coating during part of its life cycle (we'll get to that in a minute), Cryptosporidium, commonly known as Crypto, can survive for up to 10 days in the pool, even one that is properly maintained. According to the Centers for Disease Control and Prevention, in recent years Cryptosporidium has become the leading cause of swimming pool-related outbreaks of diarrheal illness in the United States.

Any time an infected person passes any amount of feces into the water, they're also releasing the parasite. At this point in its life cycle, Crypto is encased in a hardy, thick-walled shell, a form known as an oocyst. When a swimmer somehow swallows or inhales water that contains an oocyst, the parasite enters the body. It then moves through the digestive system and into the cells lining the gut, where it begins to multiply rapidly. Up to 100 million oocysts can be released with a single bowel movement.

It's not just swimming pools that are at risk when used by people infected with Crypto. Water parks, hot tubs and recreational fountains can become contaminated as well. So can lakes and the ocean, although there, with the vastly larger volume of water, the odds of contact are somewhat reduced.

Ironically, the very chemicals used to keep swimming pool water clean can themselves become health hazards. The scent of chlorine, particularly in indoor pools where adequate ventilation can be difficult, is a sign that chloramines, a compound present in chlorinated water, has turned to gas. Chloramines can cause nasal irritation, coughing or wheezing, and can even trigger asthma attacks.

Your best defense is insisting on good pool etiquette.

-- Choose a pool that insists patrons take a pre-swim shower.

-- Make sure that anyone with diarrhea never goes into the pool.

-- If you have young children, take them on frequent bathroom breaks.

-- Remember: Diapers don't belong in swimming pools.

-- Any time you see feces in the water, or when the smell of chlorine becomes strong, tell pool management.

(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.)