After a video emerged appearing to show Hillary Clinton struggling to stand upright before stumbling into a car at a 9/11 memorial ceremony in New York City Sunday, the presidential candidate was forced to go public with the pneumonia diagnosis she received two days before.

But the near-fainting spell was in fact due to dehydration, exacerbated by her condition. Her husband Bill Clinton said Monday that it wasn’t the first time. “Rarely, but on more than one occasion, over the last many, many years, the same sort of thing’s happened to her where she just got severely dehydrated,” he said.

Politico reports, citing unnamed sources, that Clinton is chronically dehydrated, and that her reluctance to regularly drink water has become a “source of tension” between the candidate and her staff. “She won’t drink water, and you try telling Hillary Clinton she has to drink water,” a source in close contact with the Democratic candidate said. Politico described a hectic rehydration mission which included lots of bottles of water as well as Gatorade.

In 2013, CBS reported that some 75% of Americans may be functioning in a chronic state of dehydration, many mistaking the symptoms for other illnesses. Whether she’s among them or not, the Democratic nominee might be better off taking a big gulp next time a staff member approaches with a glass of water. Here’s everything you need to know about dehydration:

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Why do we get dehydrated?

Dehydration occurs when your body loses more fluid than you take in, upsetting its balance of salts and sugar, which affects the way it functions. Water makes up between 60-70% of average adult— and it’s important to keep levels stable. Water is essential for life and our body uses it in many different ways, from lubricating joints and regulating our temperature to enabling waste removal via sweating, bowel movements and urination.

If not treated immediately, chronic – or ongoing – dehydration can lead to serious medical complications; it can affect your kidney function, increase the risk of kidney stones and lead to muscle damage and constipation. This level of dehydration requires hospital treatment where a sufferer will be attached to a drip to restore fluids. Of patients treated for kidney stones, 50% are likely to have a recurrence within ten years.

How do we get dehydrated?

Usually, by not drinking enough fluid – usually water – to replace what you lose. Factors that can contribute to dehydration include the climate, physical exercise (particularly in the heat) and diet. Chronic diarrhoea, vomiting, frequent urination and profuse sweating, for example from a fever, can also lead you to become dehydrated.

How can you tell you’re dehydrated?

Early warning signs include tiredness, a dry mouth, having dark coloured, strong-smelling urine, feeling thirsty and lightheaded and needing to urinate less than usual. If you begin to suffer from extreme thirst, lethargy, sunken eyes, no tears when you cry, little or no urination, a rapid heartbeat and breathing, and dizziness when you stand that lasts for more than a few seconds, you should get medical treatment straight away as these are signs of chronic dehydration.

As hydration regulates the body’s process of temperature control, being too hot or too cold can also be a sign of dehydration.

Who is most at risk?

Everyone is prone to dehydration, but some are at higher risk than others. Infants and children are particularly prone to becoming dehydrated as their bodies are made up of 65-75% water.

Elderly people are also at risk, as you lose your sense of thirst dramatically as you age, meaning older people often forget to drink enough water. Clinton, who is 68, could fall into this category. According to recent research by the BBC, one in five older people living in care homes does not drink enough fluid and those with dementia were six times more likely to be dehydrated.

People living with medical conditions, including diabetes, cystic fibrosis and kidney disease, as well as alcoholics, are also more prone to dehydration.

What’s the best way to avoid it?

Drink plenty of water! The Institute of Medicine recommends that women from the age of 19 to 70 drink 2.7 liters – or 11 cups – of beverages per day, and men of the same age are recommended to have 3.7 liters. This includes standard tap water as well as tea, coffee and juice – more or less all drinks which contain water. The U.S.’ Centers for Disease Control advises citizens to choose water instead of sugar-sweetened beverages, recommending teachers to educate students about the danger of consuming too much caffeine, including energy drinks.

Write to Kate Samuelson at kate.samuelson@time.com.