It is unlikely that the world will ever be free of disease. Most diseases are caused by very prolific microorganisms whose response to the pressures medicines exert on them is predictable: they quickly evolve immunities to those medicines while maintaining their power to infect and even kill humans.
The statement "It is unlikely that the world will ever be free of disease" most accurately describes the role played in the argument by the claim that it is a generalization used to predict the response of microorganisms to the medicines humans use to kill them.