Dear Doctor: What is the link between smoking and bladder cancer? My urologist says smoking is one of the main causes, but until I got diagnosed with bladder cancer earlier this year, I had never heard about that connection.

Dear Reader: Your urologist is correct -- decades of studies have shown that smoking cigarettes is the greatest single risk factor for developing bladder cancer.

Bladder cancer is the fourth-most-common cancer in men, and it is less common in women. Of the 80,470 new cases of bladder cancer predicted for 2019, three-fourths will occur in men. The average age for diagnosis is 73, and the disease is less often seen in Latinos and African Americans.

Although earlier research suggested that smoking doubled or tripled an individual’s risk, the most recent data indicate that cigarette smokers face four times the risk of developing bladder cancer than do non-smokers. A study conducted in 2011 connected cigarette smoking to half of all cases of bladder cancer that arose in both women and men.

To understand this connection to cigarette smoking, we should first talk a bit about bladder function and anatomy. The bladder is a hollow and flexible organ that is part of the urinary tract, which is the body’s system for getting rid of waste and excess fluids. Each day the kidneys in adults filter up to 150 quarts of blood, which produces between 1 and 2 quarts of urine. In addition to water, salt and nitrogen products like urea, urine contains numerous additional waste products and impurities that the kidneys have stripped from the blood. The urine then moves from the kidneys to the bladder via a pair of tubes known as ureters. When the bladder begins to reach its capacity of 1.5 to 2 cups of urine, the brain receives signals that we recognize as the familiar urge to urinate. Urine exits the body via a duct known as the urethra.

Among the impurities filtered out of the blood by the kidneys are the thousands of byproducts, many of them toxic, contained in cigarette smoke. These include arylamines, which are known to be potent bladder carcinogens. And even as the number of smokers in the United States has dropped, cases of bladder cancer among smokers have increased. This has led researchers to draw a connection to changes in the array of chemicals used in the manufacturing of cigarettes.

When it comes to the chemical byproducts of smoking, they do their damage at least twice. The first time is in the respiratory system, particularly in the lungs. The second time is in the urinary tract, where they move from the kidneys to the bladder, where they are held in solution for hours at a time.

For people who have quit smoking, there’s both good and bad news. Smoking cessation will reduce your bladder cancer risk. But the data shows that former smokers still have double the incidence of bladder cancer of nonsmokers. Unfortunately, exposure to secondhand smoke has been linked to increased risk as well. We have had a number of questions about bladder cancer and will continue the topic, including symptoms, treatment and outlook, in an upcoming column.

(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: When my husband and I both got sick last month, he was diagnosed with a cold and I was diagnosed with the flu. What’s the difference?

Dear Reader: Both a cold and the flu are respiratory illnesses caused by a virus. The difference between the two is the type of virus that is involved.

The flu is caused by the influenza virus, of which there are four types -- A, B, C and D. The seasonal epidemics that make millions of people miserable each year are caused by human influenza A and B viruses. The type C influenza virus typically results in a shorter and milder illness and is not associated with epidemics. Influenza D viruses primarily affect cattle and are not known to infect or cause illness in people.

The influenza A virus is further broken down into subtypes based on the presence of two proteins on the surface of the virus. These are hemagglutinin (H), which has 18 known subtypes, and neuraminidase (N), which has 11 known subtypes. To make things even more challenging for epidemiologists, many different combinations of the H and N proteins are possible. That means influenza A viruses can be further divided into different strains, like H1N1 or H3N2. Although influenza B viruses are not broken down into subtypes, they are divided by lineage and strain.

Many viruses can cause the symptoms of the common cold, but the culprit in more than half of all cases is the human rhinovirus. As with the influenza virus, viruses that cause a cold can hang in the air for up to several hours in the mist of microscopic droplets that are released when an infected person coughs or sneezes. Breathe them in, and you can become infected. Similarly, when you touch something that has been contaminated by a virus, like a door handle, dishware or even a piece of fruit in the produce section, and then touch the soft tissues of your eyes, nose or mouth, the virus can enter your body.

Viruses are basically bare-bones genetic factories that exist to hijack cells and force them to quickly reproduce the virus in great numbers. As your immune system becomes aware of the presence of the intruders, it mounts the defense that we recognize as the symptoms of a cold or the flu. This commonly includes a sore throat, mucus production, cough, sneezing, fever, headache and body aches. Each of these arises from the immune system’s efforts to create an inhospitable environment for the new virus copies that the body has been forced to produce, and to expel them from the respiratory system.

Because there is so much overlap in symptoms, it can be hard to tell the difference between a cold and the flu. A cold typically inches in, symptom by symptom. But the flu hits hard and fast, with headache, body aches, fever and exhaustion, each of them often severe. The flu is also more likely to lead to complications that require prescription medications, or when serious enough, can lead to hospitalization. The only way to definitively diagnose a case of the flu is through a laboratory test.

(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: My grandson is always carrying around a little device that he smokes like a cigarette. It puffs smoke just like one, too. I’m a former smoker, and my lungs aren't in great shape, so I’m worried. What is he using? Is it safe?

Dear Reader: Your grandson is vaping, which means he’s inhaling an aerosol produced by the device you’ve seen him using. It’s that aerosol, or vapor, that gives the practice its name. As for the specific type of device he’s using, it’s difficult to say without a clearer description. It’s definitely not an e-cigarette; those look just like tobacco cigarettes, and you would have recognized it.

E-cigarettes were the vanguard of vaping devices and first made an appearance in this country in 2007. They quickly gave way to vape pens, which as the name suggests, are slim devices that resemble a ballpoint pen. Larger and bulkier in size are vape mods, which are modified (again, there’s the root of the name) vape pens. These are known for their abundant vapor production. More recent to the marketplace is the JUUL, a small vaping device that’s about the size of a USB drive.

Whatever the specific type of device (there are many), most use disposable pods or cartridges that hold a liquid, or e-liquid, typically propylene glycol or vegetable glycerin. These liquids can contain quite a few other compounds, but at this time, manufacturers are not required to divulge them.

When heated, the liquid produces the vapor that the user is inhaling and exhaling. Most e-liquids contain nicotine, along with flavorings such as strawberry, banana, chocolate or mint. The allure of those flavorings to young people has been particularly controversial, and the outcry has resulted in a number of actions at the state and federal levels to limit their availability.

In asking whether vaping is safe, you’ve touched on a roiling debate. The general consensus is that vaping is safer than smoking cigarettes. That’s because rather than the tens of thousands of compounds produced in the burning of tobacco and its additives, many of them toxic, the number of ingredients in e-liquids is limited. However, many teens and young adults who would not otherwise have taken up smoking are finding the tech and the flavors of vaping so alluring that the practice has become widespread. It is estimated 20 percent of high school students and 5 percent of middle school students have used vaping devices, more than a third of them unaware that the product contains nicotine.

The fact is that nicotine poses a real health risk to young people, whose brains and bodies are still developing. Likewise, the effect of regularly inhaling vaporized glycerin and propylene glycol is not yet known. The practice of vaping is so new that we have to wait in real time for the results of studies into the long-term effects. So far, studies have linked vaping to hypertension, increased heart attack risk, slow wound healing, lung inflammation and increased likelihood of moving on to smoking tobacco. The Food and Drug Administration states vaping is not safe for young people. We wholeheartedly agree, and would go even further to end the sentence after the word “safe.”

(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.)