It was only when an ophthalmologist examined her eyes in detail that the answer was found. The tiny grains of iron discovered in the cornea were a vital clue to her problem - too much iron. She had been taking large supplements of iron because she was unwell and because her iron count had been down, but neither she nor her doctor had realised what effect the extra intake was having.
By taking it she had been unknowingly overriding her own immune system, which had been trying to reduce the amount of iron in her body to kill off an infection.
According to Dr Eugene Weinberg, a microbiologist and an international authority on iron in the body, her case is one of thousands of examples of a growing trend where medication is used as a quick fix rather than letting the immune system deal with it. It's a practice that has been implicated in problems as diverse as impotence, diabetes, miscarriages and cancer.
When infection or another invader strikes, the immune system goes into action, orchestrating a defensive response to the bugs, germs, bacteria and viruses.
"The immune system is basically a mixed bag of cells; organs, especially the spleen which keeps an emergency store of red and white blood cells; glands such as the lymph and the thymus; and blood vessels that ward off viruses and bacteria,'' says Adele Puhn, a medical biologist and the author of The Five Secrets for a Healthy Life.
In the constant battles to get rid of the invaders, the immune system has a huge repertoire of strategies tailor made to cover most threats. The problem is that when the immune system goes into action, the response it creates is often mistaken for the malady.
A cough, pain, runny nose, sneeze, rash, vomiting, inflammation, sickness, sore throat and diarrhoea - these are all symptoms of the immune system in action and are part of the cure. But they are often taken for the problem itself and, instead of giving the immune system time to get on with it, medication is taken, undermining the natural defences.
"We take more drugs than we need, we are a drug-orientated society. In many cases the solution our immune system provides will be more long lasting if we don't interfere with it," says Dr Herbert Dupont, the chief of internal medicine with the University of Texas at Houston.
He has investigated the use of antibiotics to prevent travellers' diarrhoea and found they do cause problems: "We know that antibiotics given early in an infectious disease will interfere with the immune response. Giving people antibiotics to prevent travellers' diarrhoea means they can eventually get a more severe form of the disease,'' he says.
Vomiting can also be an immune system response, a way of expelling toxic agents from the stomach to stop them penetrating the rest of the body. Some researchers have suggested that morning sickness in pregnant women may be the mother's immune system protecting the foetus from potentially harmful toxins. In support of their theory they say that the rate of miscarriages is lower in women who suffer morning sickness than in those who don't.
A cough is the immune system's tactic for getting rid of foreign material from the lungs and air passages, while a fever is the way to boil off some bacteria and viruses. Pain and inflammation are the warning signs that something is wrong, while a raised temperature and a runny nose are the immune system's way of trying to flush out the common cold virus that likes to colonise in the colder temperatures in the nose.
The immune system works at less obvious levels too, and its ability to withhold food from invading bacteria is one of the more subtle. The body needs iron, but it only needs so much. Some people are particularly at risk of iron overload because they have a genetic predisposition to storing it. One theory is that this genetic trait evolved in northern Europe where hunting was seasonal. In the leaner times of winter when food was not plentiful, the body needed to evolve a way of stockpiling iron.
Iron is important because it is involved in the formation of haemoglobin in the blood and in the development of muscles, and supplements and injections are given to people with low iron counts. But too much can be toxic and has been linked to a variety of problems.
Bugs that infect the body also feed on iron and the low iron count can be a sign that the immune system is working, rather than that the body needs more iron.
"When we have an infection the bacteria needs iron to thrive, so the liver stores it to prevent it getting to the bacteria. With the help of the spleen and bone marrow, the liver desperately tries to hide the iron from the bacteria, and then we spoil it all by coming along with supplements. I've been fighting this battle for 40 years and people are still being given supplements,'' says Dr Weinberg.
"The notion that the more iron a person has the healthier they are is a myth. You don't want the iron to be free in the body because it might stimulate cancer growths, or cardiac problems, and if it spills it can cause diabetes and impotence. The liver also tries to withhold iron from the inflammatory process to keep down growth of cancer or infectious organisms and keep down oxidant damage.''
The subtle workings of the immune system mean that they can be mistaken by even the most experienced of clinicians.
Dr Weinberg describes how one man suffering with too much iron had a catalogue of problems, some of them typical of iron overload, but went undiagnosed for years.
"He had abdominal problems, arthritis and impotence. He was diagnosed as a psychosomatic, was given shock therapy, became anxious and depressed, and his wife divorced him. It was only after years of this that his sister suggested he get his iron checked and, sure enough, that was the problem.''
The patient went on get his iron level down, was restored to good health and recovered everything but his wife.