Kaokin
the art of the turtle hermit
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goku met pikkon when he fought in the other world martial arts competition
Judo Ju-Jitsu
Well I don't really know how goku got exposed but Trunks (A boy from the future) warns goku about the virus so Trunks gives Goku medicine wich goku uses in the middle of his virus. Goku, while fighting android 19, starts enduring the symptoms of the virus wich weekens Goku and gives 19 an oppurtunity to end Goku if it wasn't for Vegeta.
2.5 billion people
Yes. During the Majin Buu saga (which proceeds the Cell saga), the Elder Kai gives Goku his life so that he may help in the efforts against Majin Buu. Before that (but still in the Buu saga), Goku was given 24 hours to be on Earth so that he could participate in the World Martial Arts Tournament.