Most people infected with viral encephalitis have only mild symptoms — headache, irritability or lethargy — or no symptoms, and the illness doesn't last long. Serious cases can cause:

   Drowsiness 

   Confusion and disorientation 

   Seizures 

   Sudden fever 

   Severe headache 

   Nausea and vomiting 

   Tremor or convulsions 

   Stiff neck — occasionally 

   Bulging in the soft spots (fontanels) of the skull in infants 

Urgent signs and symptoms may include altered levels of consciousness. In infants, the key signs are a stiff neck and a bulging in the soft spots of the skull. In older children, watch for severe headaches, lethargy, confusion and sensitivity to light. In adults, mental disturbances may be more prominent.