What Causes Viral Fever In A Child?

hello, my son is 7 months old n his fever started last night, i didnt chk his temp cz i thot hez teething so itmyt be dat but thn over da night it was not cuming down so in the morning i gav him calpol n he had slight runny nose and cough, but thn i took him to the docs n she sed its viral n keep him cool take his cloths off n let him cool dwn wid regular temp chk n calpol every 4 hrs, n av to take him 2mrw morning, is ther anythn 2 worry about n how long does viral fever last n wat causes it....