Why do we have an immune system?
Basically you would rot from the inside out, as bacteria and viruses destroy your cells.

Fever is your body using heat to denature the viral proteins (viruses don't have chaperone proteins like us so they are more vulnerable to heat) Sore throat is you body killing infected cells before the virus can reproduce inside them, normally the virus will reproduce inside the cell until the cell bursts or simply runs out of resources and dies both release huge numbers of the virus. 

Check out the virus life cycle. https://en.wikipedia.org/wiki/Viral_life_cycle