The Queen is being treated in hospital for gastroenteritis, an infection of the stomach and large intestine.
The two most common causes of the illness in adults in England are the norovirus and food poisoning, according to the NHS. Common symptoms include vomiting, diarrhoea and dehydration.
It is not yet known if the Queen is suffering from viral or bacterial gastroenteritis but it is unlikely she is being given antibiotics as this prolongs the time it takes to settle the condition.
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