So, I'm planning on running a RPG based in an apocalyptic world. The world will function basically equivalent to our own, except that sometime in the future, humanity created a superpowerful virus, which they then used in biowarfare. This virus then spread, killing at least 95% of humanity. My question is: How long would it take for humanity to create this virus? Basically, around what time period would we have the technology to create such a virus?
We Have Already Figured This Out
In the 60's and 70's scientists were working on bio-weapons, but it was decided that breeding super plagues was not a very smart weapon idea because there is not really any way to control what the disease will do once it has been released. It was also very VERY difficult to convince the public to financially support such a horrifying weapon, in addition we really didn't want to scare our enemies into funding such development either. If inventing nukes taught us one thing its that inventing a super weapon is a double edged sword since its only ever a matter of time before others figure out how to make them too. subsequently all funding for offensive bio-weapon research was halted and redirected towards defensive studies (epidemic response, vaccination creation, etc etc.)
We could have already bred weaponized super plagues decades ago but our leaders (and our people) decided it was not an ethical or logical choice to pursue such technology. In short, the tech has not failed to arise due to some technological or scientific shortfall, but because we chose not to create it. Humans are some real bastards, but some ideas are too dark and terrifying even for us to want to pursue them. There is no technological reason that your fictional world with our equivalent technology couldn't whip up some real nasty super-bugs. I say bugs plural because no single bug is ever going to wipe out 95% of our race, viruses and bacteria change and mutate too much too quickly, (typically towards lower lethality too) and our collective immune systems are remarkably robust at responding to plagues. But, unlike in nature, your slightly more morally bankrupt government is not limited to one plague at a time. They could drop several plagues at a time in each dispersal to guarantee that a maximum amount of people die.
Think a bio-bomb that disperses super flu, super smallpox, super cholera, and super Ebola all at the same time. The chances of a person being naturally immune to or avoiding exposure to all of those at the same time is very very small. By itself a super plague would be considered very impressive if it managed to wipe out half of a population (the black death that culturally and economically sucker-punched medieval Europe backwards a few hundred years only wiped out 30% to 40% of the overall population.) So, unlike our present timeline where our government refused to develop such diseases and instead decided to fund how to treat them, your fictional timeline they went right ahead and developed a dozen or so different weaponized plagues back in the 80's and decided to release them all at once present day.
Historically smallpox had a 30% kill rate, but some kinds of pock distribution had 90%+, specifically hemorrhagic and flat-type pustules.
Since inoculation stopped with eradication only people over 50 years old would have any immunity so it might be expected to hit the general population as badly as it hit the new world where some estimates put losses around 90% of total population.
Our world is very interconnected. A lot depends on the ability to move people and good around the world quickly and easily. If something deadly came along that would stop in a heartbeat, even before the first person died the global economy might fall apart as countries panic. If even a quarter of the world died from the virus the secondary problems of societal collapse would probably kill a lot of the remainder.
Wearing even the lightest of tinfoil hats the American and Russian scientists studying smallpox for the last 50 years may have made 'improvements'. The most obvious would be to make it not react to cowpox antibodies, so valuable time would be lost restarting production of the known vaccine. Giving it a longer incubation time, making it more infectious before displaying symptoms and more likely to have the more fatal symptoms might also be expected and make it much more of a problem.
Or with current technology AIDS or cancer probably could be made to spread through a flu strain, which could infect huge swaths of the population before being detected, and not really be possible to treat at that scale.
When you're talking about high-mortality viruses, Ebola quickly comes to mind. Even in the outbreak that happened between 2013-2016, mortality is estimated at 70% among those who were not given specialized care. That Wikipedia article lists four outbreaks during the mid 1990's. It's not too hard to imagine a nation like North Korea somehow managed to get a hold of a sample of the virus, and then used their concentration camps as a test bed for breeding a more deadly version of the virus.
It wouldn't take too much to make the virus significantly more deadly, anyway - if you could increase the period during which someone doesn't have symptoms but is still contagious, as well as increasing how easily it spreads, then it could easily kill off a significant portion of the population. From there, just follow the advice of the accepted answer for what to do with the bodies and have the rest of the deaths come from the breakdown of infrastructure.
It's fairly plausible that such a virus could be bred within a decade or two of initial acquisition of the Ebola virus, so any time after 2005 or so would be plausible for an Ebola derivative to cause a global pandemic.
Biological warfare is as scary as hell. It would take very little time for a reasonably intelligent enemy to come up with a superbug that would be capable of decimating the worlds population. We are looking at a timescale of months if these diseases do not exist already.
But let us stop and think. Any biological weapon is a double edged sword it is going to attack your own population as much as your enemies. With mobility in the world being as high as it is this effect will be impossible to control.
The populations that will do best are those that understand disease vectors and spread. The movie "Contagion" is a good model for this. Stop unnecessary interaction. Disinfect and protect where interaction is necessary.
This does not protect us against the lone psychopath. The combination of intelligence and zero social conscience gives us the area from which this attack is most likely to come.