Hi,
Gastroenteritis is what you would normally call stomach flu. Basically, it can be bacterial or viral and will cause stomach pains, vomiting and diarrhea. Sometimes you will get a fever too.
Has your brother been abroad recently? He may have a parasitic infection that may need antibiotics to help clear it, or with bad cases of gastroenteritis, you need IV fluid for rehydration and possibly meds to help with the nausea such as Compozine. Possibly antibiotics too, so that bacterial causes are covered.
Hope this helps?
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Gastroenteritis 
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