If you don't have a 'temperature', that means you don't have a fever. And thát means your body isn't trying to fight off bad bacteria as hard as it does at other times.

Having a fever is pretty much a sign of your antibodies and good bacteria eliminating the bad bacteria, causing your temperature to rise.

I'm guessing that while it may make you feel like shit, what you have isn't necessarily something your body wants to get rid of as fast as possible.

I have no idea about what kind of viruses would always give you a fever, and what kind would never though.