Firstly, that's a fictional chromosome, so they can make it whatever they want to serve the movie plot.
As for the movie, the 24th chromosome was suppose to give humans super powers in the first place, not turn them into monsters. And that's what she realized, that it actually works, but it depends on the person (not capable of being infected = not everyone will turn into a monster)
From The movie wikia page
The purpose of the new chromosome was to transform individuals into superhumans, with enhanced strength, agility, and endurance. The Olduvai scientists experimented with the chromosome, injecting it into convicted murderers. However, instead of becoming superhuman, the test subjects transformed into hideous monsters, and the scientists lost control of the facility.
Samantha has come to the conclusion that what the chromosome does to a person depends on whether or not that person has certain tendencies; essentially coming down to whether they are inherently "good" or "evil"; good people become superhuman, as planned, but evil people become mindlessly violent monsters, who instinctively "infect" other evil people with the chromosome and murder good people.