My 58 Y/o Sister Has Had A Fever That Reaches

My 58 y/o sister has had a fever that reaches 102 degrees daily and hasn't broken since last Sunday. She has had vomiting, diarrhea, a headache, and generalized weakness since that time. She has not eaten and has not been able to keep fluids in. She went to the ER on Friday night to get 2 bags of IV fluids and was discharged after they put a heart monitor on due to low potassium levels. She has enough strength to reach the bathroom and that's it--getting progressively weaker. She is a lifelong smoker but now too weak to smoke. She has severe rheumatoid arthritis for which she takes Enbrel, and she has hypertension. Could this be something other than viral gastroenteritis?