Unfortunately, fever & contagiousness are unrelated. Some contagious illnesses - such as the common cold - may never cause a fever, while others will take a while for a fever to show up. Viruses, with or without a fever, tend to be contagious. They can be spread in many ways - commonly, they are spread through droplets of saliva expelled while talking, coughing or sneezing. These droplets may contain virus even before one has developed a fever.
Some examples of contagious illnesses that can be spread before a fever develop include:
Even without a fever, if you’re seeing any symptoms or know there’s been close contact with someone who is sick, make sure to take the following precautions:
Read Next: Common Incubation Periods