First off, I'm not suggesting that this would be a good policy to implement; I'm merely posing a hypothetical.
At what point would it be better to fight the growing HIV/AIDS epidemic by immediately killing all infected individuals rather than by merely preaching prevention and researching a cure?
The UN says that 38 million people around the world are currently infected; it would be impossible to quarantine them or otherwise separate them from the uninfected population, but it wouldn't be nearly as hard to require testing and then kill everyone who failed. We wouldn't get everyone, but we could get 90%, say, and drive the rest into seclusion and self-exile/quarantine.
Obviously this is a horrible thing to contemplate, and I want to repeat again that I'm not suggesting that we're nearing the point where such a policy is necessary. I'm asking, how bad does AIDS have to get before we do reach that point?
Maybe the answer is "never". I don't know enough about the disease and its spread to say whether or not it is self-limiting. I know that I'm not at risk, but I think that the vast majority of people in the world have sex with multiple partners and are in some danger of infection. As infection rates rise, others may adopt my behavioral traits and abstain from sex outside of marriage, thereby creating an asymptotic ceiling for the disease -- but where is that ceiling? Will the disease max out at 20% of the population? 50%? 90%? We can only guess. Once the infection rate gets too high, it will become more and more difficult to kill those who are infected, simply due to their increased numbers and their likely resistance.
Assuming there is a maximum infection rate that is somehow self-limiting, is it also self-sustaining? Once the maximum rate is attained, will it be maintained across generations? It seems likely that AIDS greatly reduces reproductive opportunities, so the behavior/psychological traits that lead to AIDS may be selected against (nurture/nature) quite strongly. Again, one can only speculate.
There isn't much thought about this type of solution because not only is it awful to contemplate, it's also probably thought to be unnecessary. After all, we may have a cure soon. But what if we don't? What if the infection rate keeps rising and there's still no cure in sight? At what point do we "pull the trigger" (literally) for the sake of the species? Or do we ever?
Some may say no: individuals shouldn't be sacrificed for "the species". On the other hand, killing 38 million infected people now could save unknown millions of lives later, and those lives aren't just "the species", they're real individuals as well. Do we have a responsibility towards them?
If so, part of that responsibility surely rests with those who are infected. They have a responsibility to prevent themselves from infecting others. But what if they refuse to? What if it's impossible to educate them and prevent them from spreading the disease? Should passing the disease be a capital offense? Should such a law actually be enforced with vigor? Is it possible to enforce such a law in the mostly-lawless third-world countries where AIDS is most prevalent?
When it comes to quick-killing diseases like Ebola -- or movie scenarios like in Outbreak -- we see nothing monstrous about isolating the doomed and allowing them to die. It's tragic, but there's often no other option; if the disease can't be treated, the infected cannot be allowed to roam freely and infect others. Why is there a different standard when it comes to AIDS? Because those who are infected can easily avoid infecting others? That may be true in America, where most people have an understanding of the disease, but the same thing cannot be said for other countries, and education efforts often fail for cultural reasons.