Thanks for the additional information,
Well from the symptoms that your describe it seems like that your brother is suffering from the stomach flu or what we can refer to as viral
gastroenteritis. This involves the intestine which leads to profuse watery diarrhea without blood or mucus in the stools. The diarrhea in turn leads to the loss of electrolytes and water, which if not replenished can lead to the tiredness, headaches, fatigue
and aches in neck. This can in turn lead to the numbness and weakness in legs as well due to loss of potassium.
Now the way to go about this is to load up on electrolyte rich fluids and prevent dehydration at all costs. He shoudl be on the BRAT diet which consists of bread, rice, applesauce and toast until the diarrhea stops. But the most important thing here is to maintain his hydration status. this is very important component of therapy.
Now if he develops any fever, blood or mucus in stools, any abdominal pain, then he has to see a doctor to get antibiotics because that will mean a bacterial infection.
The second possiblity which i believe youre worried about is the swine flu H1N1 can present with just diarrhea and headaches plus cough etc but without any chest or upper respiratory tract infections the chances are less. The fact that he works in the retail industry means he comes to into contact with a lot of people. This is possiblity which would need to be ruled out with testing for the virus. The likelihood of this however is less than the above diagnosis.
So i would suggest that you get him to try my recommendations and if he doesnt get better in a couple of days take him to your doctor for testing and further treatment. I hope this helps
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