serious vs life-threatening

serious and life-threatening both are adjectives.

Word	Noun	Adjective	Verb	Adverb
serious	No	Yes	No	No
life-threatening	No	Yes	No	No

As adjectives, serious and life-threatening are synonyms defined as:

serious and life-threatening: causing fear or anxiety by threatening great harm

Other synonyms of serious include dangerous, grave, grievous, severe.

serious (adjective)	life-threatening (adjective)
concerned with work or important matters rather than play or trivialities	causing fear or anxiety by threatening great harm
completely lacking in playfulness
causing fear or anxiety by threatening great harm
requiring effort or concentration; complex and not easy to answer or solve
of great consequence
appealing to the mind