In 2027 a meteor hit the Earth's atmosphere and broke up.  Air currents carried dust from that meteor all over the world.
A few days after the meteor impact, there was a planet wide epidemic of an unknown, flu-like virus.  Those that caught the virus, however, gained super powers after the 24 hour virus had run its course.  Weak at first these powers quickly grew in strength.
A week after the virus outbreak began, a Greek virologist working for WHO, named Dr. Kreos, discovered that some people had a virus similar to those strains which gave people powers.  This virus, however, didn't give you an immunity to ever other similar virus as the others did.  THough it didn't give you any powers, it made it so that you could become infected with any other number of similar viruses, possibly gaining multiple powers.  He infected one of his test subjects that had the "Meteorus Kreos" virus with one of the other viruses, and soon that person developed the ability of that virus.
There is a saying, "Power corrupts, and absolute power corrupts absolutely."  These viruses proved it true.  Super powered humans began to terrorize the people around them.  The military and police forces of the world were ill equipped to handle them, and many smaller countries have fallen to groups of super-powered people.
Realizing that the Kreos virus could create far more powerful individuals, Dr. Kreos took his findings to the U.N. Security council.  With his guidance, they created a branch of the security forces to deal with super-powered threats with super-powered people.

Viruses have a theme, like lightning.  Though the original strain of the virus is very basic in what it lets you do, the virus is evolving, giving people new ways to use their abilities.  The virus doesn't make you immune to other strains of itself, just other viruses that give you powers, so you can gain new ways of using your powers from other strains of your virus.