Well, I wanted to post that theory for a while, now I finally have the time.
Little is known about Kabuto's past:
Kabuto was adopted by a jounin of the Yakushi clan, which is Konoha's best medic nins clan. He was found as a child in an enemy country, and he was one of the few survivors in the battlefield.
My first guess is he was found during the Third Great Shinobi War which happened when kakashi was 12. And since kakashi is 8 yrs older than Kabuto, it means Kabuto was 4 yrs old
What a 4 yrs old boy is doing on the battlefield and how is he supposed to survive? (Maybe because he is the jinchuuriki weapon of that country! But that's not the matter of that thread.)
Only one hint is revealed about his past outside of Konoha: "Bellflower", the place where Kabuto was found is called "Bellflower pass". Why would the author state such a detail. I knew it had to be a hint so I started searching about Bellflower meaning...
Suddenly I was like zOMG when I found out that the bellflower second name is "Dane's blood".
Dane killed in battle? Could this be a mere coincidence? I don't think so.Plant Lore:
The Clustered Bellflower is an alien species, originating in Europe. The nickname Dane's Blood came from the English and a belief that the flowers sprung up from the blood of Danes killed in battle.
We very well heard of a "Dan" killed in battle during the Second Great Shinobi War, that was Tsunade's lover. All of a sudden, a belief smatched me: Dan and Kabuto silver hair, Tsunade and Kabuto medical skills, it all made so much sense! Kabuto is possibly the hidden son of Tsunade and Dan!
So I started to look for more evidence:
Kabuto and Dan:
Doesn't the resemblance strike you like hell? If Kabuto removes his glasses and release his hair, he's the spitting image of Dan!
Kabuto at the age of 19 is 176cm and 65kg, so he will likely exactly reach Dan's height/weight on his prime!
Kabuto and Tsunade:
Tsunade said that Kabuto's medical senses possibly surpassed hers on her prime! Such a feat would make so much sense if Kabuto was her son.
Btw why Tsunade left Konoha and refused Jiraiya's proposal already? Once again it would make so much sense if the reason was her being secretly pregnant from Dan. She possibly left to a foreign country so that no one would know about her having a child. And knowing Tsunade, she's the type to leave a child in a foreign country.
Dan is A, Tsunade is B,
The genetic result of AxB is most of the time AB
Kabuto is AB, no comment