DEAR DR. DONOHUE: If you get through an infection with the West Nile virus, are you immune to a repeat infection? Is there a blood test that can tell if you had it in the past? Is there a blood test that tells if you have it now? – J.A.

West Nile fever deserves some background information. It’s an infection of birds and mosquitoes. Humans are incidental hosts. They are not the virus’s preferred target. Birds serve as an incubator for the virus. Humans do not.

In North America, the first case of West Nile fever occurred in the city of New York in 1999, so there is not a lot of experience with it. It has spread quite rapidly, and our experience has grown.

Only about 20 percent of infected people ever develop any symptoms. For those who do have symptoms, prominent ones include headache, muscle pains, an elevated temperature and sometimes a skin rash. Just about all weather this kind of infection without any trouble.

Out of every 150 infected people, only one will come down with serious illness. In those cases, the virus infects the brain and the brain coverings, the meninges. These people have bad headaches. In the early stages, their hands and fingers might tremble. Then larger muscles begin to shake. Such patients often become lethargic, and some slip into a coma. Fatality rates are high. Most serious infections take place in older people.

There are blood tests that can reliably differentiate a new infection from a past infection. The test for past infection generally remains positive for life.

Having had one infection confers immunity against repeat infections. The immunity is believed to last for life.

DEAR DR. DONOHUE: What is the difference between Claritin (loratadine) and Benadryl (diphenhydramine)? Both claim to be for allergies, and Benadryl is also used as a sleep aid. – M.F.

Both drugs are allergy drugs. They’re antihistamines.

During an allergic reaction, histamine is released by cells. Histamine causes itching, sneezing, nasal dripping and eye tearing. Antihistamines block the action of histamine, so goodbye itching, sneezing, nasal dripping and eye tearing – at least, that’s the hoped-for result.

Benadryl was one of the first antihistamines available to the public. It is a “first-generation” antihistamine. First-generation antihistamines have a sedating effect, something that is not advantageous during the day but is appreciated by those unable to fall asleep at night.

Claritin is of a different generation. Its claim to fame is its lack of sedation. People can take it during the day without feeling an urge to nod off.

DEAR DR. DONOHUE: I have been using a particular brand of vitamin and love it. My stool turned black. My doctor had me go through a series of gastrointestinal X-rays, and nothing harmful showed up. The doctor told me that the iron in my vitamin pill turned my stool black. I went home and checked my vitamin. It has 18 mg of iron.

I would like to continue taking the vitamin. Can it be harmful? – P.B.

Black stools have many causes. Blood oozing from a stomach or duodenal ulcer can turn the stools black. The black color comes from digested blood cells as they move through the tract. You have had your entire digestive tract examined, and nothing was found. You can cross blood off the list of possible causes.

Pepto-Bismol turns stools black. It contains bismuth, and the bismuth imparts the black color. So does charcoal, an ingredient of many gas-control medicines. Black cherries and bilberries are examples of foods that turn the stool black.

Iron can impart a black color to the stool. The recommended daily dose of iron for women from age 19 until menopause is 18 mg; for women after menopause, 8 mg; and for men age 19 and older, 8 mg. The upper limit of iron in a day is set at 45 mg. You are not going to poison yourself by taking the vitamin you like. Black stools, when they are not the result of bleeding, are not a concern.

DEAR DR. DONOHUE: My problem is, the last couple of months my stool has been the width of a pencil. I do not have diarrhea or any other digestive problems. Is this something that happens with old age? I am 81. – I.I.

A change in the caliber of stool is a sign that must not be ignored. It is not something that normally happens with aging. It might indicate cancer.

The colon frames the abdomen on three sides — on the right, across the top and on the left. Cancers in the right part of the colon tend to bleed. The bleeding is often microscopic and not recognized, but it eventually leads to an anemia. Colon cancers on the topmost and left sections of the colon can hinder food passage and cause crampy pain and, at times, obstruction of the colon. Cancers at the terminal portion of the colon, the rectum and sigmoid colon (“sigmoid” is from the Greek letter “S,” and the sigmoid colon is S-shaped), can produce bleeding and pain during a bowel movement. They can also narrow the stool’s circumference. See your doctor.

DEAR DR. DONOHUE: Advise us on a taboo subject – sexually transmitted diseases in men’s mouths. My friend, who led a very promiscuous life, does not believe this can happen. Thirty years of doctor exams, MRIs, CT scans, biopsies and drugs have not alleviated pain on one side of his mouth. – F.S.

Many sexually transmitted diseases can appear in the mouth after oral contact with genital infection. Gonorrhea, for instance, can cause a severe and hard-to-treat sore throat. The syphilis sore can be found in the mouth. Genital herpes can also infect the mouth and throat. These oral infections also apply to women.

Thirty years of repeated examinations make the diagnosis of a sexually transmitted disease most unlikely.

Dr. Donohue regrets that he is unable to answer individual letters, but he will incorporate them in his column whenever possible. Readers may write him or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475.

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