Beloved British monarch Queen Elizabeth II was hospitalized Sunday, after two days of suffering from the symptoms of a nasty stomach infection known as gastroenteritis.

The diagnosis, first reported by Buckingham Palace on Friday, has led the 86-year-old monarch to cancel a two-day trip to Rome scheduled to start Monday, as well as a number of appearances, as she recuperates.

Gastroenteritis is often mistaken for the "stomach flu," according to the National Institutes of Health's National Institute of Diabetes and Digestive and Kidney Disease in the United States, and it is identified by a range of symptoms -- including diarrhea, vomiting, abdominal pain, fever, headache and chills -- that are most often associated with influenza and other more well-known stomach bugs.

"Gastroenteritis is an inflammation of the lining of the intestines caused by a virus, bacteria or parasites. Viral gastroenteritis is the second most common illness in the U.S.," the institute wrote. "The cause is often a norovirus infection. It spreads through contaminated food or water, and contact with an infected person. The best prevention is frequent hand washing."

The institute went on to explain that "most people recover with no treatment," though the effects can be more dramatic in elderly populations.

The top-flight medical staff tending to Queen Elizabeth II's health this week at King Edward VII Hospital in London will likely direct much of their focus to ensuring that she ingests enough fluids, as dehydration is one of the biggest problems associated with gastroenteritis.

WebMD writes that most people are able to quickly and easily recover from gastroenteritis by ensuring that they get enough fluids a and slowly returning to a normal diet.

"But for others, such as babies and the elderly, loss of bodily fluid with gastroenteritis can cause dehydration, which is a life-threatening illness unless the condition is treated and fluids restored," WebMD explains.

As such, it is very important that doctors keep an eye on Elizabeth II's hydration levels to ensure she doesn't lose too much fluid.

The National Health Service of Britain told the Associated Press that the most common causes of adult gastroenteritis are food poisoning and norovirus, which is a common stomach bug that between 600,000 and 1 million British citizens suffer each year, mostly during winter months when people spend most of their time indoors.

Dr. Christopher Hawkey of the University of Nottingham's faculty of medicine and health sciences told the AP that if the queen's physicians decide she is not retaining enough fluid, she will likely need to undergo intravenous rehydration, if she isn't already.

"Not everyone can keep up with oral hydration so it is pretty routine to go to hospital and have a drip and wait for the thing to pass and keep yourself hydrated," Hawkey said. "It's very infectious and strikes in winter because people are indoors and it spreads more easily."

Gastroenteritis is most often caused by viruses such as norovirus, according to WebMD, and in such instances its effects usually pass after a couple of days. But bacterial gastroenteritis -- which arises from bacteria such as E. coli and salmonella -- can last for more than a week, and can be a greater threat to a patient's health. It is not yet publicly known what caused the queen's current bout with gastroenteritis.

The announcement that Elizabeth is in hospital has come as a surprise to many Britons, as the queen is known for her resiliency and ability to avoid health issues. Having ruled since 1952, she hasn't been hospitalized since 2003, according to a spokeswoman who spoke to the AP, though she did cancel an event several months ago due to an issue with her back.

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