We've got people here who can help you out with your storyline, but to tell the truth your story is too broad and honestly just doesn't work.
Here's some stuff that should be covered in ANY survival horror-based game.
-Origin of the specified mutation (Resident Evil had the T-Virus, I Am Legend has the fucked up cure for cancer, etc.)
-Possible effects of the mutation (Pulling from Resident Evil, the T-Virus, G-Virus, Progenitor Virus, etc. had different effects depending on the beings infected ranging from simple biological degradation [Zombies] to full on mutation [HUNTERS, Cerberus, etc.])
-Actions taken by the government towards containing the virus (cities bombed, cures researched, martial law, etc.)
-Effective measures for disposing of the infected ("Destroy the Head" is the easy one)
-How the mutation is spread (Bites, air, water, etc.)
-Reasons for the characters taking their actions (Military, civilians attempting to survive, etc.)
-Location where key events take place (The main setting and sub-settings revolving around the infection and those responsible)
Here's an example one for you, note that I'll be effectively pulling this out of my ass:
The year XXXX, a pharmaceutical company contracted by the government to create super soldiers has created a parasitic entity meant to enhance the host body's physical performance. After numerous failed trial runs, the group is finally successful and creates the first generation of these parasites they call PUPPETEER. The research continues, expanding to further improve the parasite and create the ultimate bioweapons. One day, the labs are struck by a blackout and the safety mechanisms holding back the parasites fail and these vicious monsters are released upon the lab workers. Declaring a state of emergency, the government evacuates the uninfected from the facility and orders for the complete destruction of the labs. At first the bombings seemed successful, but that was before it was discovered that the bombs made everything much worse. The parasites began to transform the environment around them to suit their own needs, the various flora and fauna becoming vicious creatures of nightmarish power. The parasites have a hive mind, cut off the leader and the rest shall fall. You are but one of many mercenaries hired by the government to destroy the core of the hive. The mission is simple: survive or die.
The Hive Parasites: Vulnerable to the cold, the parasites change their environment to increase heat and thus increase their own metabolic rate. The parasites are effectively brainless and attach to a target's spine to take over their Central Nerves System. Those who are infected normally lose all brain functions and their bodily tissues rot as they are used as food by the parasite, though many parasites can converge upon a single entity and merge, creating vastly more dangerous creatures. The parasites are controlled by a single "brain" that communicates through infected ground. The parasites cannot function upon uninfected ground and lose contact with the "brain".
Just something to throw out there...it's better to add your own creativity to a survival horror game rather than just go with the absolute basics or something that's been tapped so many times it's just not funny.