DEAR DR. DONOHUE: For two years I went to various doctors trying to find out why I was so sick. Finally I developed a skin rash and went to a dermatologist, who found out I had lupus. Since then I have been trying to get more information. Would you write a column on it? – N.M.

ANSWER: Lupus is an illness that is neither common nor rare. Many people have a friend or relative who has it. It picks mostly on women, and it commonly attacks between the ages of 20 and 45, the time when women are having children. For reasons that are not understood, lupus targets more blacks than whites. Black patients outnumber white patients by a factor of 4 to 1.

It appears that lupus results from a misfiring of the body’s immune system. The immune system attacks many of its own organs and tissues. Skin rashes are common. A rash highly suggestive of lupus is one that appears on the face. It causes redness of the cheeks and has a horizontal band that crosses the bridge of the nose and joins the cheek rashes. The entire rash forms a butterfly silhouette.

In addition to the skin outbreak, joints often hurt and swell, hair can fall out, and painless ulcers can appear in the mouth. The heart and kidneys are other lupus targets.

Sunlight exacerbates lupus skin rashes.

Antibodies in the blood confirm the lupus diagnosis and are evidence of the immune system’s involvement. One, the anti-nuclear antibody, or ANA, provides fairly strong proof that a person’s signs and symptoms stem from lupus.

In times past, lupus was often a diagnosis that carried an unfavorable prognosis. Today it is still a serious disease, but medicines can usually control the symptoms and allow patients to carry on a normal, active life in most instances.

DEAR DR. DONOHUE: I recently had a bladder infection that was treated successfully by my doctor. A friend of mine drinks cranberry juice to prevent bladder infections. Is there any truth to the protective effect of cranberry juice? – T.Y.

ANSWER: Urinary tract infections are very common in women, and most of the time those infections are bladder infections — cystitis. The principal symptoms are pain when urinating and the need to frequently empty the bladder.

Women are infected more often than men because their urethras — the tube that drains urine from the bladder — are shorter than men’s. Bacteria can enter a woman’s urethra and reach the bladder without much trouble.

Cranberry juice can prevent bladder infections. It contains a mystery substance that stops bacteria from clinging to the bladder wall, a prerequisite for developing a bladder infection.

Cranberry juice cannot cure an infection once one has started.

DEAR DR. DONOHUE: Can you tell me what causes brown spots to appear on the backs of my hands? I am in my 70s, and I don’t drink or smoke. Why do some people have these spots, and others don’t? – E.

ANSWER: If you look carefully, you’ll see that most people in their 70s have those brown spots on the backs of their hands (and elsewhere). They are called solar lentigines. The brown spots consist of skin cell clusters containing an overabundance of melanin, the pigment that imparts the brown color.

Age and sunlight are their causes.

They are not signs of any serious trouble. They have nothing to do with the liver, even though they are erroneously called “liver spots.” The acne cream Retin-A is one way of lightening them — if you wish to do so.

DEAR DR. DONOHUE: My 29-year-old grandson has floaters in his eyes. The doctor says there is no cure and doesn’t know what causes them. My grandson has become very nervous, worried he might go blind. This has depressed him. What can you tell me about this? – Worried Grandmother

ANSWER: Floaters are very common. They are bits of worn-out protein or other debris floating in the vitreous. The vitreous is a thick gel that fills the back of the eye. As the specks float around, they cast shadows on the retina, making a person see small black dots, looking like gnats, that drift in and out of the field of vision.

They are annoying but are not a prelude to blindness.

If a shower of floaters suddenly occurs, it can indicate a more serious problem, such as a detaching retina. If this happens, an emergency visit to an eye doctor is in order.

The doctor has pronounced your grandson’s eyes healthy. Your grandson must stop ruminating on floaters, or he’ll drive himself to distraction.

DEAR DR. DONOHUE: No one seems to know anything about Wegener’s granulomatosis. My niece has a friend with it and is quite worried about her. We would be grateful for any information. – B.J.

ANSWER: The nose and sinuses (upper respiratory tract), the lungs (lower respiratory tract) and the kidneys are the three major systems that Wegener’s granulomatosis attacks. If you could examine involved tissues with a microscope, you would see blood vessel inflammation and granulomas. Granulomas are heaps of dead white blood cells that were killed battling an enemy that is unknown as of this date.

Patients can have symptoms reminiscent of sinus infections. Crusts often form in the nose. Lung involvement produces labored breathing. Kidney inflammation has a host of symptoms. When Wegener’s involves all three sites — upper respiratory tract, lower respiratory tract and kidneys — patients are often gravely ill.

Treatment today with a cortisone drug and cyclophosphamide yields an 80 percent survival rate at five years. In bygone days, the two-year survival rate was less than 20 percent.

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.