Antibiotics don't treat viral infections, but there are a few antiviral drugs used to treat certain viral infections. However, the best treatment for most minor illnesses caused by viruses is often rest and plenty of fluids.... Sometimes the cause of a fever can't be identified. If you have a temperature of 101 F (38.3 C) or higher for more than three weeks and your doctor isn't able to find the cause after extensive evaluation, the diagnosis may be fever of unknown origin. ..