It is unlikely that the world will ever be free of disease.
Conclusion (unlikely = opinion)
It's unlikely the world will ever be free of disease.
Evidence
Most diseases are caused by prolific microorganisms whose response to the pressure of medicine is predictable: they evolve immunities to those medicines while maintaining their power to infect or kill humans.
Evaluate
Whenever we do Main Conclusion or Role, we remind ourselves that the vast majority of the time the main conclusion will be found in the First Sentence or as some Pivot / Rebuttal in the middle.
That helps us to hear the first sentence as the main conclusion, in realtime. We're listening for whether it sounds like an opinion from the author. When we see that it's a prediction about the future (not attributed to anyone else), we know it's an opinion from our author.
If she goes on to defend / support this claim, then we know it was the conclusion.
She does. The final two claims explain how our action of treating disease with medicine just spawns newly-evolved forms of the disease that elude our medicines. Thus, we'll unlikely ever be free of disease.
Goal
We're being asked about the first sentence, which is our Main Conclusion, so let's look for something that means Main Conclusion.