For years Pokerus has been around yet little is
known about this virus. It seems to have no
known effect on Humans, and on Pokemon it
is only known to boost Pokemon's attributes.
However, in the region of Johto, Pokemon have
started become very sick, this strange illness
has started to spread so rapidly around the
region, that nobody is allowed to enter or
leave the region to stop the spread to other
regions.
Yet, some Pokemon in the Kanto region has
started to show signs of this illness. It is
thought that perhaps Pokerus may actually be
the cause, however as it's wild Pokemon that
are generally found sick, and Pokerus's known
effects wear off after a period of time, it can
not be conclusive that Pokerus is actually the
cause.
A group of trainers will be embarking on their Pokemon journey's in Kanto, and will encounter said sick Pokemon. It will take their efforts combined to figure out what the cause is, if Pokerus is involved in anyway shape or form.