Diarrhea (from the Greek διάρροια meaning "flowing through"), also spelled diarrhoea, is the condition of having three or more loose or liquid bowel movements per day. It is a common cause of death in developing countries and the second most common cause of infant deaths worldwide. The loss of fluids through diarrhea can cause dehydration and electrolyte imbalances. In 2009 diarrhea was estimated to have caused 1.1 million deaths in people aged 5 and over and 1.5 million deaths in children under the age of 5.Oral rehydration solutions (ORS) with modest amounts of electrolytes and zinc tablets are the treatment of choice and have been estimated to have saved 50 million children in the past 25 years. ORS should be begun at early as possible. Vomiting does often occurs during the first hour or two of treatment with ORS, but this seldom prevents successful rehydration as most of the fluid is still absorbed. The World Health Organization (WHO) recommends that if a child vomits, to wait five or ten minutes and then start again more slowly. Homemade solutions recommended by WHO include salted drinks (e.g. salted rice water or a salted yoghurt drink) and vegetable or chicken soup with salt. If available, supplemental potassium, as well as supplemental zinc, can be added to or given with this homemade solution. It's also recommended that persons with diarrhea, if able, continue or resume eating as this speeds recovery of normal intestinal function and generally leads to diarrhea of shorter duration. Clean plain water can be one of several fluids given. There are commercial solutions such as Pedialyte, and relief agencies such as UNICEF widely distribute packets of salts and sugar. A homemade solution can be made by adding between one-half to one teaspoon of salt (about 2-3 grams) and six teaspoons sugar (about 18 grams) to one liter of water. If the person drinks solutions with too much sugar or too much salt, these can draw fluid from the body to the bowel, cause osmotic diarrhea, and make dehydration worse. In a WHO publication, it's stated that a homemade Oral rehydration solution (ORS) should approximately have the “taste of tears.”

Christian Spencer Jessen (born 4 March 1977) is an English doctor and television presenter.

He is best known for presenting Channel 4's Supersize vs Superskinny and Embarrassing Bodies alongside Pixie McKenna and Dawn Harper. As well as presenting medical programmes on television, he contributes regularly to newspapers and magazines such as The Evening Standard, The Daily Mail, The Daily Express, FHM, Attitude, Closer and Arena. Following his series Supersize vs Superskinny, Jessen wrote the accompanying book Supersize vs Superskinny: Take Control of Your Weight. He also has his own stand alone book based on the weird and wonderful medical questions he gets asked called "Can I Just Ask" published by Hay House.

Jessen studied medicine as an undergraduate at University College London, receiving his MBBS degrees in 2000. He holds an MSc degree in sexual health, and has a particular interest in HIV and malaria, which were the focus of his work for two years in Kenya and Uganda.

Alongside his television career, Jessen is a doctor in a clinic in London and has worked privately in the UK oil and gas industry - as well as for the Australian, New Zealand and Canadian immigration services. Jessen also provides occupational health for London Underground and Virgin Trains. Currently he works more in the field of sexual health and HIV.