Queen Elizabeth II, now 86, has held her throne since the age of 25. Pictured, Elizabeth arrives at a garden reception at Government House in Auckland, New Zealand, in February 2002.
Princess Elizabeth Alexandra Mary, right, poses for a photo with her younger sister, Princess Margaret, in 1933.
Queen Elizabeth II arrives at King's Cross railway station in London with her four Corgis in October 1969 after vacationing at Balmoral Castle in Scotland.
Queen Elizabeth, accompanied by her husband, Prince Philip, the Duke of Edinburgh, waves to the crowd on June 2, 1953, after being crowned at Westminster Abbey in London. Her coronation was the first worldwide televised event.
Elizabeth and Philip sit in the grass with their two children, Charles, Prince of Wales, and Princess Anne, circa 1951. They later had two more children, Edward, Earl of Wessex, and Andrew, Duke of York.
Prince Richard of Gloucester, from left, the Duchess of Gloucester, the Queen Mother, Prince Bernhard of the Netherlands, Queen Elizabeth II, her husband, the Duke of Edinburgh, Queen Juliana of the Netherlands, Princess Alexandra, Lord Snowdon and his wife, Princess Margaret, Elizabeth's younger sister, pose for a family picture in April 1972 in London during a visit to Britain by the Netherlands' royal family.
Visiting Canada in 1997, Elizabeth smiles as she leaves the Alexander Graham Bell Homestead in Brantford, Ontario.
Elizabeth leaves St. George's Chapel in Windsor Castle after the wedding of Sophie Rhys-Jones and her son Prince Edward in June 1999.
Elizabeth inspects the Canadian Guard of Honour in July 1992 before taking part in Canada Day celebrations in Ottawa.
Elizabeth sits beside Philip as she gives a speech at the state opening of Parliament in the House of Lords in November 1999.
Elizabeth's coach travels through London in 2004.
Elizabeth greets veterans at the Gurdwara Sri Guru Singh Sabha, a Sikh Temple in London, in October 2004.
Elizabeth is offered a rose by a well-wisher while visiting the Princess Diana memorial fountain in London's Hyde Park in July 2004.
Elizabeth poses in Dresden during a state visit to Germany in November 2004.
Elizabeth attends a state banquet at the Philharmonic Hall in Bratislava, Slovakia, in October 2008.
Elizabeth meets actors Sanjeev Bhaskar, second left, and Meera Syal, left, after watching a performance by Nutkhut, a London-based dance company, at Buckingham Palace in October 2009.
Elizabeth gives a speech during a state banquet for the visit of Indian President Pratibha Devisingh Patil at Windsor Castle in October 2009.
Elizabeth receives flowers from a Girl Guide after attending the Christmas Day church service at St Mary's Church in Sandringham, England, in 2010.
Elizabeth visits St. George's School in Windsor, England, in May 2011.
Elizabeth looks at a horse at the Windsor Horse Show in Windsor, England, in May 2011.
U.S. President Barack Obama toasts with Elizabeth during a state banquet at Buckingham Palace in May 2011.
Queen Elizabeth, from left, Prince William, and Catherine, Duchess of Cambridge, look up from the balcony of Buckingham Palace after the service of thanksgiving at St. Paul's Cathedral in June 2012.
Elizabeth leaves No. 10 Downing Street in London in July 2012.
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STORY HIGHLIGHTS
- Billions of people get gastroenteritis each year, experts say
- The cause is usually a virus, but in other cases bacteria, parasites can be blamed
- Symptoms generally pass in a few days but could last up to 10 days
- Doctors say to watch your fluids, replace lost electrolytes
(CNN) -- The queen of England has something that most of us get at one point or another -- but usually gastroenteritis doesn't lead to a hospital stay.
Experts say, however, that anytime an older patient gets it, the complications can be more serious.
Dr. Won Kyoo Cho, the director of gastroenterology at MedStar Washington Hospital Center, says doctors are more likely to admit someone to the hospital when they are concerned about the patient not being able to keep anything in his or her stomach.
"When they get dehydrated, it usually it gets worse. We need to get them IV fluids" to replace potassium, magnesium and phosphorous, he says.
Queen Elizabeth II hospitalized for stomach bug
Dr. Corey Siegel, a gastroenterologist and assistant professor at Dartmouth, says dehydration can lead to kidney problems.
And the elderly are at especially high risk of getting dehydrated when they have gastroenteritis, he says.
Once treated, most symptoms disappear in a few days, Cho says.
Here are five things you should know about gastroenteritis:
1. It is often called the stomach flu, but it's not.
It often can be caused by a virus, but it's not an influenza virus. It can also be caused by bacteria or parasites, but that is rarer. The National Institutes of Health says viral gastroenteritis is the second most common illness in the United States. People exhibit symptoms of watery diarrhea, abdominal cramps, nausea or vomiting, and sometimes a fever.
2. It is rarely deadly.
But infants, older adults or people with compromised immune systems can have serious complications. That's because they are at risk for dehydration from loss of fluids, according to the Centers for Disease Control and Prevention. The Cleveland Clinic says about 27 people die each day in the United States from severe gastroenteritis. But most people recover without needing hospitalization, the NIH says. Cho says more than 3 billion people around the world get the disease each year.
3. It can be foodborne or waterborne, or you can get it from another person.
Gastroenteritis is contagious and can be spread through contact between someone who is infected and another person. It can also be spread by food (especially seafood) or drink that is contaminated, but that is less common. Prevention is key, the Mayo Clinic says, advising people to wash their hands frequently and avoid food or water or utensils that they think might be contaminated. Cho says when doctors diagnose the disease, they look for two things: symptoms and circumstances. The latter can help narrow down what type of gastroenteritis a person has.
4. It cannot be treated with antibiotics -- if it is viral.
The Cleveland Clinic also says doctors usually do not recommend antidiarrheal medications because they can prolong the infection. The best course of action is to replenish with fluids or foods that contain the most electrolytes or complex carbohydrates, the experts say. The CDC recommends keeping oral rehydration solution on hand and giving it to someone who has had diarrhea. Cho says doctors usually hold off for a few days before using antibiotics. "It can be a little bit tricky," he says, adding doctors usually take a stool sample to see if the cause is bacterial.
5. It can last up to 10 days.
It depends on the type of virus the patient has, the CDC says.
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CNN's William Hudson and Elizabeth Cohen contributed to this report.