18 month old has a fever of 103.7 but acting completely fine and only has a runny nose.?

Fever. As long as child is acting appropriately and is adequately hydrated, you can observe. You should have low threshold to seek medical opinion, if high grade fever persists for more than a few days.
Use your own sense. When your baby has a high fever, you need to talk to your own pediatrician. Babies can get colds with high fever and stay home but, chceck with the child's doctor immediately.