Most show little or no symptoms, but in the severest cases the cells of the body are starved of energy.
If better-tolerated regimens existed, the market could surge as infected people with mild or no symptoms get treatment for the first time.
It is thought these cases do not represent a "tip of the iceberg" with far more people being infected with mild or no symptoms, but the infection is still being analysed.
Making the virus hard to detect is that infected poultry display slight or no symptoms, unlike the H5N1 strain which kills birds and raged across the region in the middle of the last decade.
The appearance of cases with mild or no symptoms in humans could make tracing even more difficult, but may also mean that many people infected do not get seriously ill and recover quickly, making the virus is less deadly than it appears.
He adds he has heard from some people who have used the product even with mild cognitive symptoms but no diagnosis, or even a few with no symptoms but family histories of Alzheimer's disease.
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Many people with essential thrombocythemia have no signs or symptoms.
If you don't wear glasses or contacts, have no symptoms of eye trouble, and are at a low risk of developing eye diseases, such as glaucoma, it's recommended that you have an eye exam at the following intervals.
In early AMD, people have no vision loss or other symptoms, but do have yellow deposits in the retina called drusen.
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To wait at least 48 hours if they have experienced symptoms of vomiting or diarrhoea and no more than two visitors should be at a patient's bedside at any one time.
The CDC says that although HPV is very common in both sexes, most men won't develop symptoms or serious health problems, and there is no test to detect the virus in men.
Usually after a decade or so, half of patients with this type of MS will develop secondary progressive disease where the symptoms get gradually worse and there are no or very few periods of remission.
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There is no way of telling whether you are having a TIA or a stroke when the symptoms first start.
She said that while there was no evidence foul play was involved, certain symptoms were consistent with dogs who ingest rat or mouse poison.
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