actually, the rarest pokeon that can be caught is on route 123
celebi: 1 in 20 mil
shiny: 1 in 8 thou
pokerus: 1 in 20 thou
modest nature: 1 in 25
so heres the equation:
20,000,000 * 8,000 * 20,000 * 25 =
8e16 says my calculator. but i say:
160,000,000,000 * 20,000 =
3,200,000,000,000,000 * 25 =
80,000,000,000,000,000!!!
so the odds of finding a shiny celebi with pokerus that has a modest nature are 1 in 80 Quadrillion!!!
(where 1 is thousand, 2 is million, 3 is billion and 4 is trillion and 1 = 000) its pretty complicated math, but when you get down to it, its pretty easy.