It was never supposed to get out, but of course it did. It was the last line of defense, or perhaps it was better stated to be the last line of offense. How you put it doesn't matter; what happened is what matters.
What happened is that it got out. It was nothing particularly lethal, though it did destroy almost every life that it touched. Only one in perhaps fifteen was killed outright, dying horribly in twisted fever, whimpering for release from the horror that life had become. Others died more horribly under the watchful glare of camera lights in isolation wards, but by the time they had recognized that this was indeed a plague, and a plague unknown, most of the East Coast was affected. In the Midwest, they barricaded, blockaded and finally dynamited the bridges across the Mississippi and Missouri, but the boats that plied the ports of those mighty rivers carried unwanted cargo. Still, the spread was enough slowed that the more rural of the teaching hospitals were able at last to develop a vaccine of sorts... too late to help the cities with their jetports, and the East itself was irecoverably and universally infected.
The virus was spread, at first, by exactly those who one expects to spread a virus, and through the same routes as viruses are normally spread. In children, it was a nuisance, nothing more, just a particularly nasty miserable flu. The virus was large, a filterable virus, of the sort that carries lots of information in the RNA code. Parts of it had once been experiments in transposonation, in "gene- therapy". The mechanism that had finally been devised to carry the information to all of the body's cells had been abandoned as too risky; the proteins that would split a plasmid and insert a codon all too often split the chain at the wrong place, and often absorbed a codon, to transport it to a place where it should not be. It was actually pretty well-conceived and well-implimented, but simply too dangerous for any practice that cared to call itself medicine. For the generals, though, it was satisfactory indeed.
In the basement, it mutated someone who was just a tad too careless. After all, this was not Ebola, and the secretive nature of the experimentation precluded giving the technicians adequate warning. they did know that they wre working on something with military applications; that could not be concealed. But in the absence of any symptoms in the first few days, those who had become infected spread it far and wide through their children and their children's schools. In adults, the transposonation proteins were activated by the presence of corticosterones not present in children, and while the children watched, their parents began to grow... different.
In the basement, it mutated the single-mother lab tech. She wasn't all that different at first, she simply could not abide the light, could not abide the noise of her children, could not abide the sounds of traffic in the streets and in the basement she twisted and her bones became soft and proteins were made not quite right at the deepest metabolic levels, and she died miserably, but not before packing her children off to school where they promptly sneezed the infection onto all of their classmates. By the time they came home, there was something in the basement, something somewhat amorphous, something racked with a fevered hunger which it had no mouth thereby to feed. The children, to their credit, upon discovering the foamy thing wrapped around the foot of the basement stairs, immediately called the police. The officer who answered the phone refused to send an officer, and demanded to speak to their mother. The little girl told the dispatcher that they didn't know where their mother was, but that they thought that the thing downstairs had maybe eaten her. The dispatcher hung up on them.
Mom had reached a nearly liquid state, and the temperature in the basement was rising with the outpouring of heat from what remained of her body. The little girl ran back to the top of the stairs and watched, sobbing quietly, as the thing at the foot of the stairs seemed to grope about. It was about the shape of a huge half-baked blueberry muffin that had been left behind a dumpster for a week or two, and smelled about the same, a loathsome yeasty odor with a nasty sour tang to it. The odor got worse as a bubble broke the surface of the mass like a fart-bubble in a bathtub of maple-syrup. It shuddered from time to time, and wheezed. Clutching a bread knife, the little girl tried to quiet her sniveling little brother as she watched with unblinking eyes as the thing in the basement developed an unwholesome sheen on the surface, and shivered like jello on a boombox. It began to moan, a high keening whine that rasped at the brain like the whine of a dentist's drill, and a two-lobed eye appeared on the surface, throbbing as if to the beat of a heart. The little boy sobbed uncontrollably as the thing began to try to work its way upstairs. The little girl slammed and locked the basement door, and backed towards the living room as the noisome mass began to ooze under the door. It made it about halfway through, and then expired, mostly. Deep within it, portions still lived, though, and these began to devour what remained. Fortunately for the two children, these portions were on the other side of the door. As the little girl sat mute, clutching her breadknife and her brother, she heard sounds rather like wet cloth tearing, followed by something that sounded like celery being chewed, and as she watched, the mass that had oozed under the door was yanked back under it. When she thought she saw something like a tiny clawed hand reaching under the door for a better grip, it was too much for her. She gathered her brother under her arm, and fled.
She went to the neighbor's house, and knocked. Something flopped loosely against the door. Brave in her desperation, she ran to the side window and peeked in, and a moment later, returned, blanched, to grab her brother and drag him away. He chose this moment to break into a screaming fit, and she simply knocked him down, and told him to shut up with such force that he remained silent until spoken to, unheard of in this child. As she dragged him away, the sound of the flopping against the door became more intense, and more rapid.
She ran down the block to where her brother's best friend lived, hoping that he might be alright, and they found him cowering in his tree-fort. They couldn't join him, for something slavered at the base of the tree, and had gnawed it half through. It turned something like an immense ear at them, but continued to gnaw. Its legs were short and twisted, but it seemed to have six of them. It had red hair, long about the huge ear, short over the rest of the body. It was the exact color of the hair of her brother's best-friend's big sister, the cheerleader, whose car was parked atop the fence. Something snarled behind her, and she and her brother ran. The snarling thing ran behind them, but slowly, for its legs were not fully formed. As the girl looked back, she screamed at the thing that had once been a neighbor, and it snarled back through teeth the size of carving knives as it loped slowly on its two misshapen forelegs, dragging its jointed bulbous body behind it.
The plague's alterations were most intense in the first few victims. Most of the monstrosities created by haphazard rewriting of the body weren't very functional. They might have legs and senses and something remotely resembling minds, but more often than not, the restructurings of internal organs were incomplete or grossly flawed. Most of those affected by the initial generation of the virus simply deliquesced over fairly short periods, though very commonly the virus added sufficient genetic material (derived from the hosts' introns) to the oocysts of fertile females to cause fertilization, and some of these introns carried enough information to produce forms which could parasitize the hosts directly. Clawbabies were a fairly common result, repulsive little lemuroid bipeds that literally ate their way out of the parent's body. It was fortunate that they had a lifespan measured in hours, and were as a rule sterile.
That was the good thing about the beginning of the plague, that at first, it mostly liquefied. But as the virus passed from host to host, picking up introns at random, binding them together, and reinserting the new plasmids into the protein factories of the cells of a new host, a rapid evolution began to take place...
The little girl ran on. She ran past the school bus where it lay parked on the sidewalk, with the eyeless driver yawning his fangs at her footsteps as his tentacles dragged another screaming brokenlegged child towards the bag of guts and lashing clawed cilia that sat upon his lap, oozing a foaming acid pus onto another half-digested child. The back door of the school-bus was open, and so Jenni decided that there must be other kids around. She hoped. Behind her, Johnny determinedly staggered on in her wake. He didn't even look at the screaming child on the bus, whose pleas for rescue were suddenly cut off as the busdriver finally got the kid within range of his teeth.
Jenni ran on. Johnny lamely followed, and then they were too out of breath to even try to run further. They hid behind some bushes, evidently well enough, for when the huge-toothed monster which had been dogging their trail caught up, it couldn't find them. Its forepaws were better formed now, if bloody, but the bulbous anterior section was even more distended. The thing paused, and gnashed its teeth. It hissed, a sort of sandpapery gargling sound, and seemed to be fresh out of ideas. It balanced itself on one bloody paw, and scrabbled at the scabs covering its eye with the back of the other paw. Flakes of papery dead tissues fluttered to the ground, and evidently it could see much better now with the obstructions gone from the bulbous liquid orb that covered most of what had once been a human face. It grumbled and muttered, and then dragged itself across the street where it began to struggle with the cephalopod monstrosity that had formed from the busdriver and his erstwhile dinner. The ogrish octopoid's tentacles were strong indeed, but the teeth of the other thing managed to sever one, and as the tentacle lashed like some foul living spaghetti, it gnashed and gnarled and slurped. As it ate, gobbling like the world's hungriest dog at a bowlful of innards, its wounds healed, and the stunted lower body began to fill out. As Jenni and Johnny sat and stared shocked, small bumps emerged from the rapidly-growing anterior pod, and grew into tentacles. The octopoid began to slither out of the front door of the bus, groping for the ground, dragging its bag of guts behind it as it went. Jenni vomited loudly, and the thing with all of the teeth turned its head in their direction, and began to slither towards them. Jenni wiped her mouth, wiped her hand on the grass, and dragged Johnny to his feet, and they fled, towards the one place that seemed at all safe: school.
At school, they were not alone. There was a large crowd of children, most of them older than Jenni, who was in the sixth grade. one of them, in junior highschool maybe, seemed to be the leader. She listened intently, and the consensus seemd to be that the inside of the elementary school was the safest place they could be. They all went inside, a particularly wise move now that the toothy thing was approaching. It could move a lot faster now that it had tentacles to aid the forelimbs, which were now less bloody, and covered with thick scabby scars, suckers, and claws. The tentacles seemed to be stiffening, as if cartilage was growing within.
They barricaded themselves within the school cafeteria. The thing was not able to even get inside the door of the building, which was a relief. It seemed to have forgotten how doorknobs worked. Still, it was nervewracking to listen to its grumbling and muttering as it slammed itself against the doors, and tried to pry under them with its tentacles. Soon enough, it decided to try elsewhere for tastier food. outside the windows, the children watched as various loathsome transgenic chimerae flopped, bumped and slithered across the schoolyard, fighting whenever they met. The thing with the teeth, and the redhaired monster with the big ear seemed to be the strongest, or at least the best-equipped fighters, at least until the clawbabies showed up in a chittering mass of nasty little jaws on legs. They attacked the less formidable of the monsters with simple pack tactics, like pirahna with feet, jump, rip off a bite, jump back, chew, jump again. The toothy things killed many, but the supply of clawbabies seemed inexhaustible. Eventually, they subdued the others, and picked their bodies clean, and then the ran off, looking for other food. Inside the cafeteria, the children unheld their collective breaths, and some went to sleep, while others cried or muttered to others, trying to sort out what their world had become.