Dr. Sanders, I'm still sick. The slender 75-year-old woman shook her head sadly. I hate to be a bother, but you've got to help me with this cough. It's just about killing me. Her face was pale; the skin below her eyes was dark; she'd lost a lot of weight. Whatever she had, if it wasn't killing her, it certainly was making her very sick. Her problems started a few months earlier. She came in to see me with a simple cough. She'd had a cold, but that infection was long gone. It was strange, she told me; she was fine during the day - with maybe the occasional cough - but at night, when she lay down, it was unbearable. She slept in a chair the past two nights, because in bed she kept waking up to cough. And the day before, she had a strange pressure in her chest. That worried her enough to make an appointment with me. It must have been scary, because she usually came to the office no more than once a year. It wasn't that she didn't like me, she said, she just didn't like to see doctors unless she really had to. Although she was worried at that visit, I wasn't. Her report that she spent the previous couple of nights sleeping in a chair might have concerned me about some of my patients, because one sign of heart disease is the inability to lie down flat, but this woman had no risk factors for heart disease other than her age. She never smoked, and she was in great shape. At 75, she had no medical problems; she ate well, and she exercised almost every day. She was one of my healthiest patients. Her physical exam was normal that first visit. So was her EKG. I figured that her airways were still oversensitive from the cold she had. Or maybe she had some kind of bacterial bronchitis that moved in after the cold moved on. I gave her an inhaler to calm her twitchy airways and a short course of antibiotics and expected the cough to disappear. But over the next few months, she kept coming back. Each time, her cough was a little bit worse and she was a little sicker. None of the tests I ordered were revealing, and none of the medications I prescribed helped. She was starting to get frustrated, and so was I. Seeing her nearly four months later, I was worried. She'd lost a lot of weight - her normally thin face looked gaunt. Her eyes were tired and puffy. And she couldn't get more than a few words out before a new paroxysm cut her off. She couldn't sleep, she told me, between bursts of a barking cough. And she had no appetite at all. While examining her, I found nothing. She had no fever. Her lungs were clear. There were no enlarged glands suggesting an ongoing infection. Now when she coughed, however, dark yellow mucus came up. Cough is the most common single reason for a visit to a primary-care doctor. Most of the time it's caused by an upper-respiratory infection or a cold. But that type of cough usually resolves on its own. A persistent cough has many possible causes. The most likely is an allergy. So I treated her with antihistamines. They didn't help. Heartburn (also known as reflux) can also cause a cough as the acid from the stomach irritates the delicate tissues of the throat. I tried a couple of different antacid medications. They didn't help either. She said she never wheezed, but she did have some chest tightness and that cough. Was this asthma? I gave her another inhaler. It didn't work any better than the first one. She clearly didn't have what most people have when they have a persistent cough. Could it be lung cancer? She never smoked - but 10 to 15 percent of people with lung cancer don't smoke. Could this be tuberculosis? She traveled in rural Central America decades earlier, so she could have been exposed. A chest X-ray might have shown either of those diseases, but it didn't. In fact, it was completely normal. Blood tests showed no sign of infection. I sent her for pulmonary-function tests. The results weren't completely normal, but the technician said that her constant coughing made the test difficult to interpret. Now she was back, sicker than ever, and I was starting to get nervous. I had already referred her to an ear, nose and throat doctor and a gastroenterologist. If none of this panned out, I wasn't sure what to do next. I wish I could tell you that I spent days poring over the literature on coughs. But that actually came later. A couple of days after I saw the patient, I was out running, and I found myself thinking about an old friend. This friend, a tall, willowy woman in her late 60s - knowing how I loved weird medical cases - had, a couple of years earlier, written to me about the story of her own illness. She too was an older woman who developed a persistent cough. It took her doctor weeks, but he finally figured it out: she had what was called Lady Windermere Syndrome, a lung infection that affects mostly older women - in particular slender, healthy women. I suddenly realized that my patient probably had this disease too. First described in 1992, this cough is caused by a bacterium related to tuberculosis called Mycobacterium avium intracellulare, or M.A.I. This bacterium is found everywhere, but it's not very aggressive and rarely causes illness; it's usually killed off by the body's immune system.