This is what happened. In the year 1995 the US government began to work on bio-chemical weapons. By 1998 the military scientist discovered that instead of creating a weapon that would take out the entire population of a country, they had made a virus that was very similar to the SARS virus.
They tested the virus in China and discovered that there was only a .5% death rate. Not nearly enough for something that was intended to take down a world superpower.
The millitairy continued to work on the virus. They attempted splicing the virus with the AIDS virus. The intent was to make AIDS an airborne disease. AIDS would shut down the immune system, and then SARS would do the rest.
This pushed the death rate up to 30%. once again not nearly enough for a superpower. Work continued for five more years. Every once in a while one or two of the scientist would be killed to keep the rest of them from talking. A good preventave measure, but it would slow the work down.
In the year 2003 the governmental fuckery with nature has created one of the most powerful weapons in the world. It wasn't just powerful enough to take out a small country, but enough to erase an entire continent off the face of the earth.
Over the next five years laboratory experiments concluded that the virus had a 99.8% death rate. Any dictators wet dream. The virus couldn't be killed because there was no vaccine for it. That's how bio-warfare worked. You create the weapon, then you create a cure, but if you don't it doesn't really matter because the weapon will be used in a country far away from your own. What could anybody want to do with a cure for a virus that was created to kill anyways?
The virus was 100% airborne and couldn't be killed because it mutated every ten seconds. The virus could survive without a host for two and a half years, this, the scientists never did know.
The virus worked by infecting a host through the respiratory track. 90% of the time it would look like a simple cold, And the host would be dead in about a week but not before infecting every person that the host came it contact with before dying.
9.8% of the time it would infect the hosts brain. Instead of killing the host in one week, the host would recover from the cold. However, they would be delirious and extremely hostile. They were turned into cannibals and would prey on humans during the night. Any actual survivors would think of them as zombies. They were slow, felt no pain, and it they caught you, you are pretty much good for dead. They were fairly hard to kill.
The other .2 percent were completely immune to the virus.
In the year 2011 the United States proved to the worlds most strongest superpower. The leader was growing impatient with the Iraq/Afghanistan war. It looked like the US was losing the war and the decision was made to pull out and try the new weapon.
In one week all the troops were out of Iraq and Afghanistan. A B254 bomber was on its way to drop two glass capsules which held the super virus. They shattered on contact witht he ground and the virus began its work. In two weeks almost everybody was dead of infected and spreading the plague while waiting to die.
In only two months the plague reached Russia, China, the Soviet Union, and northern Africa. The virus ripped through those countries and eliminated almost half the world in only four months. The scientists did not see this actually happening.
There was three scientists left that new the virus was created in the US. Two of them were killed to prevent any leaks, the third hung himself.
In five months, the virus plowed into America with the help of wind currents. The United States fell very quickly to the virus. In less than six months, the United States president wiped out billions of people including himself, giving a new meaning to mass genocide. The new Hitler.
The virus left roughly 13 million people out of 6.5 billion uninfected. 9.8 percent of that 6.5 billion people turned into human preying animals. They hunted at night. During the day they hid in empty factories, houses, abandoned businesses, or anywhere that was dark.
The 13 million survivors were left to live in a lawless, hostile world. Some would kill anybody that moved, while others created a small society and would try to create a small army to combat the zombies and other threats.