Many Sith can be turned, like Darth Vader, Vestara Khai ect.
I find it interesting how many more jedi turned to the Dark side than the other way around. In numerous cases, the continuation of the Sith, or at least the Dark side was caused by the jedi. In fact, going back into EU lore, it was the jedi who exiled "Dark side practitioners" who then found their way into Sith space (back when the Sith were a species, not just a side on the light/Dark coin). The more I think about it, all of the "imbalance" that the jedi have had to "correct" has ultimately come from their own ranks.
When Vader and Sidious ruled the galaxy, they ruled through tyranny, fear, slavery. Aliens were subjected to slavery the most, basically, the Empire and its rulers were 'alienists'. People were unhappy, they lived in utter fear and terror while the Jedi were absent and without power. This is not balance, whether there were two Jedi and two Sith.
Yes, Palpatine was VERY xenophobic, as was Vader, although Vader at least had how he was treated as a slave as a partial influence. Yes, almost all non-human species had a very rough time of it under Palpatine during the
twenty four (24) years of the Empire.
Now, how about all of the species that were enslaved by the Hutts during the
over twenty five thousand (25,000+) years prior to that, when the jedi were present and the Sith more or less absent? When entire worlds, entire species, no matter where in the Galaxy they were, served the Hutts as slaves. And the jedi did nothing. In fact, during some of the various wars, they deliberately turned a blind eye to it in order to use hyperspace lanes through Hutt space. At least during Palpatine's Empire, there was no one who could have done anything about it. For over 25,000 years, the slave species had to exist in a galaxy where the might of the jedi order existed....and did nothing for them.
Jedi are meant to protect innocents and serve the will of the Force. They are a selfless, compassionate, moral people while a Sith causes the imbalance by being selfish, cruel, greedy, killing and destroying everything in their path for their own gain.
In my experience, selflessness, compassion, and morality are all both subjective and situational. As are selfishness, cruelty, and greed. "You're going to find that many of the truths we cling to depend greatly on our point of view." ~Obi-Wan Kenobi.
Now, summing up everything above so far, with the body/cancer analogy, I did look up the actual prophecy regarding the "Chosen One" and interestingly Lucas also mentions the Sith being similar to a cancer. (Also, it's very difficult to find what the prophecy actually says, as opposed to what everyone thinks it says/means.)
However, the "welcoming cancer into your body" analogy is, I think, false. Cancer, to the best of my knowledge, isn't a communicable disease. It's never brought in from an external source, it's ALWAYS created
within the body. So, going back to my point above, it's the jedi that are continually causing the cancer. In many, many cases it's jedi that have broken from/left/been exiled/etc. from the jedi order that become the cancer. In fact, looking at it that way, the jedi, rather than exterminating the cancer from their own ranks, simply send it out into the body (galaxy) to grow and flourish, cause imbalance and suffering. The jedi both cause the cancer, and refuse to ever eliminate it when they have the chance.
My personal view remains that both sides must exist for there to be balance, but I think it could also be argued that the only way to ever have balance is to remove BOTH sides.