What Causes Sudden Attack Of Fits?

MY NEPHEW IS SIX YEARS OLD STAYING IN JEDDHA ON THE DAY OF HIS LAST EXAM HE SUDDENLY GOT AN ATTACK OF FITS HIS PARENTS CONSULTED A DOCTOR THERE ALL THE TESTS INCLUDING CT SCAN AND MRI ARE CLEAR THEY HAD TAKEN TEN DROPS OF HIS CSF AND SENT IT FOR THE BACTERIAL AS WELL AS VIRAL TEST THEY SAY THAT HE HAS BEEN INFECTED WITH VIRAL THING WHAT IS EXACTLY THE CAUSE FOR THIS