Scars of Mirrodin block was memorable to longtime players for many reasons: The Mirrodin/New Phyrexia setting, which recalled not one but two past planes, the artifact theme, the return of the Phyrexians... but most of all because of infect, which brought poison counters back to Magic after so many years of their supposedly having no chance of being seen again. Infect is perhaps one of the most controversial keywords ever; as many people loved it as hated it. And, according to Mark Rosewater on his Tumblr blog, both parties will get a chance to do so again sometime soon.
When koscookemonstr asked, "Will we ever see infect again?" Rosewater responded with the well-known fact:
Most likely. It was beloved and behated mechanic.
Infect was beloved because it was a nostalgic reference and an alternate win condition; it was (be)hated because it was perceived of as unfair and enabling degenerate decks and combos, because there were no answers to it, and also to a lesser extent because of its parasitic design (poison cards only work to win with other poison cards).
When the-machine-orthodoxy asked, "Do you believe that if you had more answers for infect, as you had said you were developing during your podcast, it would have been (and hopefully will be) less behated?" Rosewater said:
I think people would feel better if they felt there were more answers.
Now, in order for poison counters not to feel like a second life total, there needed to not be a way to remove them (no Leeches here). That explicit of an answer is out. Melira, Sylvok Outcast was the one direct answer to infect in the block. Of course, future anti-infect cards wouldn't want to spoil Melira's uniqueness in flavor and gameplay; but perhaps permanents are the answer, such as ones that increase the number of poison counters it takes to kill you as long as they're on the battlefield.