A sudden onset of fever without any symptoms indicates that it is likely to be an acute infection. Fever can occur when an acute infection causes an inflammatory response throughout the body. If the fever is caused by an infection of the upper or lower respiratory tract, there are usually signs of sneezing, runny nose, sore throat, or coughing or coughing up sputum. The absence of these manifestations indicates that there are most likely no respiratory-related symptoms. If the fever is caused by a urinary tract infection, it usually travels up to pyelonephritis. This can be accompanied by back pain, and if it is not accompanied by significant back pain or frequent or urgent urination, then it is not a urinary tract infection either. If the infection is caused by an infection elsewhere, the corresponding symptoms will be suggestive. If the fever is sudden and there is no indication of symptoms, it is not possible to determine from the symptoms what is causing the infection. In this case, tests can be done, such as a urine test to see if there are bacteria or a lot of white blood cells in the urine to determine if it is a urinary tract infection. In the case of fever without any accompanying symptoms, a chest CT can also be done, as some people have pneumonia, but the symptoms are not as obvious.