The strongest warrior

After the infection of a certain virus, the
death rate was reaching 30%. In a matter
of those three months, excluding the mass
deaths of animals, the population of the
world dropped by nearly 2 billion!"
"The survivors neared 5 billion, whom all
have a natural resistance against the
disease."
"During these nightmarish three months,
the whole planet experienced excruciating
pain."
"During this process, the 5 billion survivors
realized that their bodies have
strengthened. Virtually everyone's power, speed, rate of cell division, and skin
toughness have at least doubled! Even an
average man could easily surpass the
previous world champion in running and
weightlifting."
"And then... disaster started!"
"The humans who have lived so
comfortably have strengthened so much.
The beasts who have survived the RR
disease have went through even harsher
conditions. In their transformations, their
strength has raised surprisingly far beyond
humans'. And some of these horrifying
beasts got even smarter!"