För det första är det en fiktiv kromosom, så de kan göra vad de vill tjäna filmplotten.
När det gäller filmen antogs den 24: e kromosomen att ge människor supermakter i första hand, inte förvandla dem till monster. Och det var det hon insåg att det verkligen fungerar, men det beror på personen (inte kunna smittas = inte alla kommer att bli ett monster)
Från Filmswikia-sidan
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.