Dear Doctor: So many people are required to undergo tuberculosis screening even though they don't know anyone with TB and have no symptoms. Just how contagious is tuberculosis?

Dear Reader: Tuberculosis, or TB, is an extremely infectious and potentially life-threatening disease. One-third of the world's population is infected with TB, which causes about 1.7 million deaths each year. Infection rates in the United States are quite low by comparison, with 9,287 new cases reported in 2016.

The causative agent of TB is a rod-shaped bacterium called Mycobacterium tuberculosis. It's released into the air when an infected person sneezes, coughs, or even speaks or breathes. By breathing in these microscopic droplets, which can remain airborne for several hours, a person who comes into contact with an individual with an active TB infection can contract the disease.

Transmission of TB after brief exposure to an infected individual is rare. According to the Centers for Disease Control and Prevention, TB cannot be spread by contact with clothes, a handshake, the toilet or other surfaces.

Once the TB bacterium is inhaled, it can move through the bloodstream to other parts of the body, like the kidneys, spine or brain. However, in most cases, the bacterium affects the lungs. There, if left untreated, it can cause pockets and cavities to form. These can bleed, get infected and form abscesses, cause holes or block airways. Left untreated, TB can kill you.

You asked why even those without TB symptoms are tested. The reason is that, in most healthy people, the body's immune system is good at neutralizing the TB bacterium. It does this by forming a protective capsule, known as a granuloma, around the bacillus. Although this isolates the bacterium, it doesn't kill it. That means that the person is now a TB carrier, otherwise known as latent TB. The danger here is that the immune system can lose the upper hand at any time, and the latent infection can become active.

People with latent TB don't have symptoms and are not contagious. However, they must receive treatment to get rid of the disease. Without anti-TB drugs, active disease can develop. That risk is highest during the first few years following infection. For some people, it continues throughout their lives.

So what are the symptoms of TB?

They include a bad cough that lasts for three weeks or more; a cough that brings up blood or sputum; general weakness or fatigue; unexplained weight loss, fever, chills and night sweats.

If you have had contact with someone who has TB, or if you have several of these symptoms, particularly those involving the lungs, you should get tested. A blood or skin test will show whether the TB bacillus is present. A chest X-ray or a sputum culture will reveal active disease.

Treatment will depend on whether the disease is active or latent. Active disease is treated with several drugs taken over the course of up to six to nine months. A further complication is that some TB bacterium is drug-resistant. This requires very specific treatment by infectious disease specialists.

(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 are the benefits of oregano oil?

Dear Reader: Oregano is well-known for its culinary versatility -- I like it with pasta and in chimichurri sauce -- but it has purported health benefits as well. A native of the Mediterranean region, the plant was used by the ancient Greek physician Hippocrates as an antiseptic and as an aid for digestive and respiratory ailments.

The beneficial effects of oregano appear to be from two specific chemical compounds, carvacrol and thymol. Both have been shown to have activity against multiple bacteria in laboratory settings. Among the many subspecies of oregano, all of them contain these two compounds -- known as phenols -- in some proportion. Oregano contains other phenol compounds as well, potentially boosting its antibacterial effects.

These compounds could be at least part of the reason why both the oregano plant and oregano oil inhibit the replication of intestinal pathogenic bacteria, such as Escherichia coli, Bacillus cereus and Salmonella, as well as the respiratory pathogens Strep. pyogenes, Staph. aureus and Moraxella. A 2014 study looked at the effect of adding either oil of the oregano plant (Origanum vulgare), carvacrol, thymol or a control to an antibiotic regimen against the organism Staph. aureus. The oregano oil and each of the phenols increased the inhibition of bacteria when added to the drug tetracycline, but none had any effect on the inhibition caused by the drugs erythromycin or norfloxacin. Also, oregano alone and carvacrol alone have shown activity against the norovirus, which causes gastrointestinal illnesses.

As for whether this anti-microbial effect translates to the world outside of a petri dish, only one good human study shows any efficacy. It looked at 14 patients who had non-pathogenic (not linked to illness) parasites in their stool. The participants were given 600 milligrams of oregano oil (Origanum vulgare) for six weeks. The study showed reduction or elimination of the parasites with the use of oregano oil. There was no placebo portion of the trial.

When it comes to cancer, Origanum vulgare has shown inhibition of cells in both breast and colon cancer. Another species, Origanum majorana, commonly known as sweet marjoram, stops the replication of liver cancer cells within the lab. A 2017 study in the European Journal of Nutrition looked at rats that were given medications to induce breast cancer. The 75 rats were separated into three groups: 25 were given 3 percent Origanum vulgare, 25 were given 0.3 percent and the last 25 were not given oregano. After 15 weeks of the study, the group given 0.3 percent oregano had a 44 percent reduction in the incidence of breast tumors. However, the rats given the 3 percent oregano had no reduction in the incidence of breast tumors. So maybe there is a Goldilocks zone for oregano. But again, human studies are nonexistent.

Some people report nausea and bloating with oregano oil, but overall, it appears safe. A 90-day study of oregano oil in rats found no adverse effects.

As for whether oregano oil has benefits, the answer is a solid maybe. No controlled human trials can even say whether it's effective against either infection or cancer. Some of the individual components such as carvacrol and thymol should be further studied in humans.

(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: I'm living with Crohn's disease. Even though I feel fine, my wife is certain that it has caused me to be vitamin B12 deficient. If you could explain the signs of a B12 deficiency, it may put her mind at ease and get me off the hot seat.

Dear Reader: When you have Crohn's disease, a portion of your digestive tract is chronically irritated or inflamed. Symptoms include pain or cramping in your abdomen, diarrhea, fever and weight loss. The cause of Crohn's disease, one of a group of disorders known as inflammatory bowel disease (IBD), is not known at this time. However, changes in diet and the use of medications to reduce inflammation, along with bowel rest, can help relieve symptoms.

The reason your wife is concerned about your levels of vitamin B12 is that Crohn's disease can cause the tissue of the bowel to thicken, become scarred or develop ulcers. Any of these may cause the bowel to lose the ability to properly absorb nutrients. When the disease is located at the far end of the small intestine, where vitamin B12 absorption takes place, maintaining healthy levels of that vitamin can indeed become an issue.

We need about 2.4 micrograms of B12 per day for our bodies to carry out functions like making red blood cells, keeping our nerves healthy and making DNA. Sources of B12 include fish, meat, poultry, eggs, milk and other dairy products, with liver and clams delivering a particularly hefty dose of the vitamin. It's also added to some breakfast cereals and is available as a supplement.

Although most people in the United States get enough B12 through a balanced diet, up to 15 percent of Americans have some degree of deficiency. Some older adults lack adequate hydrochloric acid in their stomachs to properly separate B12 from food. That means that when the contents of the stomach reaches the end of the small intestine, B12 is not available in a form that can be absorbed. In individuals like yourself, whose bowel is chronically inflamed, absorption in the small intestine can become compromised, even with proper digestion in the stomach.

Symptoms that can indicate a vitamin B12 deficiency include feeling weak and tired for no apparent reason, mental confusion or "fogginess," a decline in balance, the onset of depression and even signs of dementia. Other indicators may be poor appetite, unexplained weight loss, ongoing constipation and nerve problems like prickling or tingling in the hands or feet. In extreme cases, a deficiency of vitamin B12 can lead to megaloblastic anemia, which can cause extreme fatigue, dizziness and pale skin.

If these symptoms sound familiar, your family doctor will measure your B12 levels via a simple blood or urine test. Should a deficiency be found, treatment may include a diet of foods rich in B12, regular B12 supplements or, in a severe case, B12 injections. And considering that long-term B12 deficiency can cause nerve damage and other serious problems, we would also prescribe a "thank you" to your wife.

(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.)