I want to know if somebody spits in your mouth, does your dna kill their saliva or does their saliva stay there forever?

Interesting question: Human saliva is 99.5% water, while the other 0.5% consists of electrolytes, mucus, glycoproteins, enzymes, and antibacterial compounds such as secretory IgA and lysozyme. Dna does not kill saliva. Firstly, the water component of saliva would be absorb or pass through, where as the rest of the content would be treated as foreign body and would be destroy by your own defense mechanism.