How dangerous is a fever of 140 degrees fahrenheit?

You would be dead. A fever of 140 does not exist - you would be dead. A fever of 104 is a significant fever, worthy of finding out why, but not in and of itself dangerous. A fever of 100.4 is a low grade fever, the type you may see accompany many minor illness like colds and sore throats. Remember, though, fever is just a symptom - it is not a disease itself. It signals something else is going on.