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!"