I keep thinking body snatchers but I’m not sure I remember its a mom and a son but the sons immune to whatever changes then like the ones who are infected will infect others by spitting in their drinks and when u fall asleep is when it happens the moms infected so she stays awake and has to inject herself because she almost does and she’s keeping her son safe from her ex husband whos infected and is coming after the son
You’re sure it’s not the Nicole Kidman version, The Invasion? http://www.imdb.com/title/tt0427392/