If you watch the Naruto Anime, then you might enjoy this. I was sitting at home watching Season 4, when something hit me like a ton of bricks. It started in episode 91 and ended at episode 96.

It turns out "Grandma"  Tsunade made a bet with Naruto about whether or not he could master the Rasengan in a week. If he loses, he has to admit he was wrong about becoming Hokage, and he loses all his money in fat "Froggy" that he worked so hard to earn (he gambled with scratch cards or something and got it on the first try). If he wins, Tsunade will admit he is worthy of becoming Hokage; and as a bonus, tossed in her Grandfathers(the First Hokage) Necklace.

Shizune later explains to Naruto that every person who has worn the necklace has died, except Tsunade(this being her little brother Nawaki, and her late boyfriend Dan). I later went back and studied this, and I found to my astonishment that it wasn't the necklace that was getting them killed.

If you watch closely, you'll see that everytime Tsunade gives the necklace to someone, she kisses them on the forehead. So I came to the conclusion that her kisses is what is slowly killing them(I don't know if its poisonous or something, or if her lips give off a natural slow working neuro toxin or what)(it killed her little brother the next day). But I ran into a stump as to how Naruto hasn't died. I then studied a bit more and found in Naruto Shippuden that the Nine Tails Chakra acts like a poison to normal people, Naruto however is immune to this effect(despite being burned). So in conclusion, the immunity to the NIne Tails Chakra and the fact that the chakra itself heals Naruto every time he is wounded, prevents her venomous kiss from even giving Naruto a side-effect of the poison.

That is a great theory! Next they will be putting her saliva on all there ninja weapons as its such a deadly toxin. Which will wipe out everyone apart from Naruto and Tsunade. So Naruto will get his wish to be Hokage.  Shame no one will actually be there apart from a 60 year old woman with serious anger issues

sonicmcmuffin : I know. But how many people is Grandma Tsunade going to kiss? And chances are, Killer B will live as well. I don't know about the Raikage though. Still, I believe Naruto's immunity to injury and Tsunade's demon kiss is due to the Nine Tails Yang(Negative, Red, Spiritual) Chakra. But to have that kind of power, comes with consequences(Naruto after being in Four Tails Form, his skin looked like he went past a tan and on to a burn, Third Degree burn if I may).

Wow that is an excellent theory I have never thought of that and I did see Grandma Tsunade kiss them on the forehead but I thought that wasn't the reason why they died.

mike345 : It's a slow acting toxin. Both Dan (her boyfriend) and Nawaki (her brother) died by other shinobi's hands. I figured the toxin was like a drug and slowly dulled the senses to the point where they couldn't fight. That is where Naruto comes in.
We all know he is a bit of a dim wit, and his senses are dulled anyway. However, the nine tails seems to make up for all Naruto's faults. In episode 44 of Naruto: Shippuden (translation: Accel) Yamato is surprised that Naruto is even alive after going four-tails on Orochimaru's rear end, because the effect it had on Sakura after she had already healed her arm was like a poison. So let me make some points:
1. Every person Tsunade has kissed on screen has died except Naruto.
2. After that kiss, Dan and Nawaki die to wounds that would doubtfully have stopped them, they both had a dream, and both seemed to carry the will of fire.
3. Naruto already carries the biggest burden and power in the world, but he also suffers the most consequences when tapping in to that power via the nine tails chakra burns his skin and causes him to bleed severely.
4. Naruto also shows that he is stronger than the nine tails by not only suppressing that vast amount of chakra, but say if he does gain full control over the nine tails yang chakra, and he is able to release the suppressing chakra, not only would he have superb chakra control, but he may have more chakra then the nine tails himself.
5. Naruto is being wounded by the nine tails chakra and healed by the nine tails chakra, which would suggest that he may have the nine tails yin(physical) chakra inside him, working for his benefit and the yang(mental) chakra working against him.
6. Naruto has yet to be wounded beyond his ability to heal.
7. Tsunade is a medical ninja, so she may have accidentally gained a contagious disease accidentally or something.

So from this we get that Tsunade's kiss does have a part in the deaths of Dan and Nawaki; but Naruto is a fast healer and not easy to kill because he is a Jinchuriki born child (through his mother), which may also suggest why he is immune to the nine tails chakra.

That's a very good observation one that I never found out you have a very good point