A large part of this internal mess comes from the endogenous retrovirus. A normal virus works by moving into a host cell and using it to reproduce, but retroviruses reproduce by actually mixing their own genetic material with the DNA of the host cell they're invading. If a normal virus is a home invasion robber, busting down your door and smashing up your stuff, a retrovirus is the creature from Alien, impregnating you with its horrible seed and producing a twisted mockery of everything you once were, and then laughing as that atrocity murders all of your friends. Metaphorically speaking, of course.

In the distant past, retroviruses picked up by our ancestors would occasionally find their way into the sex organs, and the newly virused-up DNA was passed along to their children. As a result of all this virus-laden boning, we modern humans have about 100,000 of these microscopic gate-crashers cluttering up our DNA. When you add in the assorted genetic trash they've left behind, more than 40 percent of human DNA is made up of ancient, sinister and almost certainly cursed viruses.

Via hgu.mrc.ac.uk
Scientifically speaking, they are "the clumpy purple thingies."

But these viruses can't do much harm today, right? Oh, how we love your unflinching optimism, rhetorical question, but you're wrong again: Tests on the cerebrospinal fluid of schizophrenic patients (that's science-talk for "the brain juice of crazies") have revealed unexpectedly high levels of a particular endogenous retrovirus. This suggests that the misunderstood mental illness is, in fact, a long-term side effect of a retrovirus that we all have inside us already. It's no good locking the doors, people; that crazy is calling from inside the house.

Via Yikrazuul
"Get in the car. I just want to talk."

In people who are genetically inclined, this retrovirus can be "switched on" by a separate viral infection that occurs around the time of birth -- herpes, toxoplasmosis (aka Cat Zombie Disease) or even plain old influenza. Studies have found that babies born in winter months -- around flu season -- are at greater risk of developing not just schizophrenia, but bipolar disorder and multiple sclerosis later in life, suggesting that all three conditions might just be different reactions to the same retrovirus. Basically, if you catch a cold as a baby, you could end up building bombs in a shack to combat the lizard people who've infiltrated our government, and it's all thanks to the insanity that lives in everybody's blood.

Under certain circumstances, long-dead DNA sequences can suddenly return to life, giving their human carriers an all-you-can-catch buffet of horrible diseases. A common form of muscular dystrophy, FSHD, is caused by a "dead" gene present in all humans. But it's only "dead" because it's missing one specific sequence that allows it to be successfully transcribed. All it takes is one mutation, and the gene rises from the grave to wreak its terrible revenge on humanity.

Getty
Above: One of the hot ones.

Scientists know about FSHD because it's easy to study -- the condition can be traced back to a single, dominant gene. But it's no fluke: A gene thought to put people at risk for Crohn's disease was resurrected after being "dead" for about 25 million years. The cause of the resurrection? Another retrovirus. In other words, prehistoric killers are not only living in your skin but also can team up together, Voltron-style, to bring you down from the inside. One researcher -- we'll call him Dr. Foreshadow -- said, "Don't count a gene out until it's totally gone from a genome."

We assume he spun his chair around immediately afterward to ominously add "and maybe not even then" before disappearing in an explosion of smoke and villainous laughter.

A group of scientists have been reconstructing the Neanderthal genome from fossilized bones recently, and when they compared their results with genomes of various humans from around the world, they discovered that most modern humans in Europe and Asia share between 1 and 4 percent of their genome with Neanderthals -- a trait that is not found among people from sub-Saharan Africa. In other words, if you're reading this and you have any European or Asian ancestors at all, you're technically not fully human and therefore must be destroyed before you can infect others.

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It's theorized that the interspecies lovin' took place back in the Middle East about 60,000 years ago, after early humanity had left some of its cousins behind in Africa. In fact, considering how long humans have had to mix with each other since then, the original human-Neanderthal genetic crossover might have been as much as 10 or even 20 percent. Neanderthals weren't the only interspecies bonees, either: In 2010, researchers discovered another species, the Denisovans, and to the surprise of nobody, we apparently stuck it in them, too.

Scientists think that about 10,000 years after the Neanderthal mixing took place, a group of humans came across the Denisovans in South Asia and thought, "Hey, any port in a storm." A few cooed grunts, sexy sax solos and millennia later, and presto! What parts of you aren't undead or a virus are probably subhuman.

Wait, put the gun down! No, not because there's so much to live for; you're a monster and you need to be purged. We're just not done yet. Not by a long shot.

We've mentioned before that your father's smoking could have made you fat. What we didn't mention, though, was that your grandparents' actions could also have doomed you to a short and unhappy life. Man, what did you do to piss off all of history? 'Cause it sure seems to hate the crap out of you.

Via Wikimedia Commons
Do you owe Napoleon money? Dude, just pay it already.

A study in Sweden revealed a strange pattern in a rural community that had gone through periods of both famine and abundance in the 19th century. The study found that the grandsons of men who'd had childhoods coinciding with abundant years -- i.e., the ones who had stuffed their faces with grain for a season or two -- had a life expectancy of 32 years less than the grandsons of those who had experienced famine, with the deaths caused mainly by diabetes, heart disease and presumably the shame of having extremely fat grandparents. Daughters with gluttonous grandmothers suffered a similar fate.

Photos.com
Though we admit that eating enough to kill future generations is pretty damn impressive.

Luckily, this delayed-reaction DNA sabotage doesn't always have to be negative: Mice exposed to "enriched" learning environments developed an improved memory that was passed on to offspring that had never experienced it. Man, it seems like just about anything you do with your life now has a serious impact on the genetics of later generations. So it's a good thing you're snacking on broccoli and stimulating your mind with such an esteemed source of learning on your way to save that orphanage, right?

iStockphoto

The assassin bug of South America lands on the faces of sleeping humans and sucks their blood while pooping on them at the same time, proving once again that nature is a sick and murderous pervert. Wait, did you just have a seizure? Weren't you reading an article on DNA? What the hell are we doing talking about bugs? Oh, God, there aren't bugs in your DNA, are there?! Relax, relax, it's nothing like that: It's just a parasite that lives in the poop floating in your bloodstream.

Via CDC
Above: Ew.

See, when the bug's victim scratches the bite, the crap sitting on the wound enters his system. And since assassin bugs carry the parasite T. cruzi, you get a free bonus prize with your s**t-blood: Chagas disease. This condition can severely damage the heart and digestive system, producing, among other things, a symptom known as "enlarged colon." It's responsible for about 20,000 deaths a year, mostly in South America, but it occasionally pops up in the U.S. as well. Researchers who deliberately infected chicken eggs with T. cruzi and then tested the offspring of the infected chickens that emerged found that not only did those chickens have the parasite DNA, but so did their offspring, and so on.

Photos.com
And all of their omelets tasted like s**t.

But keep in mind that T. cruzi isn't necessarily the only parasite in our DNA, just the first we've discovered. Your cells might literally be swarming with the stuff. Hey, it's survival of the fittest. You know the old saying: If you can't beat 'em, take a dump in their veins and live forever in their children. If you don't believe us, just ask Charles Darwin. Oh, wait, you can't: Chagas disease is thought to have been the illness that killed him.

Via Wikimedia Commons
Also, he died over a hundred years ago

Regular Cracked readers probably know by now that babies are capable of murder in the womb, because that's our end goal: teaching you things that you can never un-know. But even womb murder isn't the end of the story. In some cases, you can end up absorbing your newly dead twin and having its DNA live on inside you, a condition known formally as "chimerism" and informally as "what happens when God stays up late watching movies by David Lynch."

Who just happens to be a Chimera himself.

In 2002, a woman named Lydia Fairchild submitted DNA tests for her three children as part of a welfare claim, only to have the results prove that genetically, she wasn't the mother. Since DNA is considered the gold standard of medical evidence, she was accused of somehow stealing the children, even after the poor woman gave birth to another "nonrelated" child right in front of a social worker. Finally, more extensive testing unlocked the mystery: Her ovaries had a different set of DNA than her bloodstream. In other words, she'd given birth to her dead sister's children. And then, presumably, she never stopped screaming.

Photos.com
"And where's your 'friend' at now, sweetie?" "She's inside me, Mommy. She's inside me."

Again, that's no fluke: In another case, a woman getting typed for a kidney transplant found out that one son was genetically hers, while two more belonged to her similarly dead sibling. A teenage boy being treated for an undescended testicle turned out to be carrying an ovary on that side from a twin sister -- as if an undescended testicle wasn't going to get him made fun of enough, now he's half ghost-woman as well? Jesus, screw you too, genetics. Chimerism is thought to be rare but also massively underdiagnosed, since it's undetectable outside of DNA testing, which doesn't happen to normal folks all that often. Potential symptoms can include slightly different-colored eyes, uneven skin pigmentation and waking up at night to find 'YOU KILLED ME' written on the bathroom mirror just before being strangled by your own reflection.

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