What would the best way of doing it be? I am assuming a quick spreading, quick killing illness that humans wont have time to reasearch and find a cure for.
Or is there any other better way that will guarantee success to the aliens.
Why the hell are you giving them ideas? 
Reported for forum change.
Obviously not a GQ.
But the problem with a “quick spreading, quick killing” virus is that the two criteria are in opposition to each other. The problem is that once someone dies of the virus, they stop spreading the virus. So all you have to do is quarantine the infected people until they die, and the rest of the population is safe. This is why Ebola outbreaks are serious, but not sustainable. If you get infected with Ebola you get very sick very quickly and are likely to die. You are extremely contagious during this period, shedding infected blood and bodily fluids everywhere. Then you die, and stop shedding infected bodily fluids everywhere. How many people did you infect before you died?
This is why Ebola, despite being incredibly infectious, is not actually quick spreading. Anything that kills quickly and the epidemic will quickly burn out, preventing the disease from becoming pandemic.
So yes, you can have a devastating plague that kills lots of people. But you’ll still have to move in and mop up the survivors conventionally.
Poison the wine and beer.
Some genetic-linked disease that causes sterility should do the trick. The lack of immediate lethality will give it time to spread around.
So we’d be left with teetotalers, devout Muslims, and people like me who never drink wine or beer! Yay?
Shoot them all for sport … they’re easy to smell out … and none too bright either.
Start with a vicious whisper campaign, see how many they can off themselves and then it should be easy to slaughter off the remaining. I think the key is leaving enough during the process so they’ll bury most of them, so lay off the backhoe operators until last.
Nitpick: That isn’t really true with Ebola. Corpses are still contagious, and people were getting infected preparing their relatives for burial. Your overall point stands though.
Moderator Action
This is more speculation and opinion than fact, so off to IMHO it goes.
Moving thread from General Questions to In My Humble Opinion.
Oh and by the way, that book “To Serve Man” is a cookbook. Don’t say I didn’t warn you.
Seriously, this. Sure, you bomb the big cities first, to destroy the population concentrations and to annihilate the high-tech civilization. But from there on out, it’s vermin control. You have to put squaddies on the ground with hand-weapons.
It’s one of those cases where “nuke 'em from orbit, it’s the only way to be sure” is not correct.
ah, well, the solution is to have something ELSE spread the quick-killing virus around. Engineer some nano-drones to run around spreading the human-specific virus, or the quick and dirty method is to just spray the planet with the virus.
Bombing will kill humans and other living creatures. The question asks how to kill only humans.
Self-replicating killer robots. Perhaps microscopic robots (nanites) that are programmed to spread and infect every human, then kill us all at the same time.
Tailor a virus that is slow acting and hard to detect that targets human propagation (I’d go with something that targets males and females separately and in different ways, if possible, that way even those who slip through might not be able to find compatible mates to have children). Even if you are only 80% successful you will wipe out a huge swath of the population within a generation or so.
If anyone survives then you can issue those hunting licenses with no limits.
What’s the difference between a nanodrone that infects someone with a virus and a virus? Serious question, because that’s what a virus is. A real-world nanondrone that infects you with bad code to force your body to make copies of itself.
If all humans were to suddenly die it would be an apocalyptic nightmare for the ecosystem and many animals. This has been studied and covered in books like “The World Without Us” by Alan Weisman, and in TV shows like “Life After People”: Life After People - Wikipedia
Deprived of human oversight and without an electric grid to run safety systems, most of the approx. 437 nuclear reactors on earth would likely melt down or eventually catch fire and explode. Similarly the spent fuel pools at each reactor site after boiling away the cooling water would probably catch fire. All over the earth, gigantic plumes of radioactive smoke would fill the sky. It would be like a thousand Chernobyls, but with no humans to contain it. The total contaminated ground footprint would be vast.
Similarly, many of the 700 petroleum refineries worldwide would eventually on fire and burn. It would be like when Saddam Hussein set fire to Kuwait’s oil wells, only a thousand times worse and without any human to put them out. This isn’t speculation but is the best estimate by refinery operating personnel.
The approx. 2,400 oil tankers worldwide would eventually break up and empty their petroleum into the oceans. It would be like 2,000 Exxon Valdez disasters, and with nobody to clean it up.
Many chemical plants would rupture or explode, spewing vast amounts of toxic waste into the environment. It would be like the Bhopal India disaster except instead of humans dying it would be animals, and thousands of locations on earth.
Deprived of human care, most zoo animals and many domestic animals would perish outright. The surviving domestic animals would come under attack from unrestricted large predators. Eventually nature would reach a balance, but not before most domestic animals were wiped out.
In short, if all humans on earth suddenly died or vanished, it would render the earth a toxic, radioactive waste. Domestic and wild animals would be devastated. It would not be a planet any alien in their right mind would be interested in.
So the vector is a macroscopic drone (a robot rat) that spreads smaller drones (robot fleas) that infect all hu-mons with a tailored virus (bubonic plague).
The good part about this plan is it doesn’t rely on human-to-human transmission. The problem is keeping the virus from jumping to nonhuman species.
If they’re bright enough to master traveling here I’m sure they can figure something out.
That’s essentially what I’m about to say.
If they can engineer interstellar space travel, they can engineer some crazy hyoomon only death beam in which to bathe the planet. Or some other super scifi solution.
Two thoughts about that: First, I think some of it is exaggerated a bit, especially if you’re willing to look long-term. Oil spills naturally decompose in decades. Most radioactivity will decay in a couple of centuries and most animals can survive and reproduce even with levels of radiation that humans find unacceptable. (After all, we plan on living 80 years, and dying of cancer at 10 is not a good option for us. But it’s not such a big deal for creatures that reach sexual maturity in 1 year and die of natural causes in 5.) Some hotspots will be lethal for millions of years, but not large swaths that would affect everything.
Second, it’s not such a stretch for the aliens to land in time to safely shut down the nuke plants. In fact, if we favor the replicating nanobot invasion theory, the aliens just need to program some bots to poison us, and some bots to take over our machinery. This sort of one-two punch makes sense even without the worry of environmental disasters; we’re a lot less likely to avoid/fight off the poison nanobots if we have no power, no cars and no computers.