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Michael: Forget your Hans Grubers, and Lord Voldemorts, and Jean-Baptiste Emanuel Zorgs — it’s time to meet some real supervillains. Well, supervillains as far as the human body is concerned. They’re called retroviruses, and they actually change their host cell’s DNA.

You’re probably familiar with viruses: they’re super small — roughly a thousand times smaller than most bacteria, which themselves are much smaller than most of the cells in your body. These microbes are essentially a floating protein shell with a little bit of DNA or RNA inside, which are the molecules they use to store their genetic information.

The virus only attaches to certain types of cells, depending on the type of virus. Once it finds the cell it needs and latches on, the virus releases its guts into the cell, where it then uses the machinery of the cell it’s infected to make more viruses.

Ribosomes in the cell, which are basically the cellular manufacturers, read the viral RNA and start making the amino acids the RNA says to make. Those amino acids become enzymes, which create protein shells and more viral DNA and RNA, which then come together to form more viruses. The new viruses pile up until they explode out of the cell, releasing more viruses into the body.

Viruses can use this process to spread really fast. Luckily for us, we can create antibodies to recognize and destroy cells infected with viruses before they overwhelm everything. Still, even regular viruses can be pretty dangerous to the health of a cell.

But, retroviruses make regular viruses look like a bunch of dumb babies. Retroviruses get their supervillain status because of an enzyme called reverse transcriptase. Reverse transcriptase reads the virus’s RNA and turns it into DNA, which then makes its way into the cell’s DNA and attaches itself.

Once the retroviral DNA is in the cell’s DNA, the cell just starts copying the new DNA along with its own. After the retroviral DNA infects a cell, it can lie dormant for a while before it activates itself. On top of this, these viruses are reproduced a ton, so it provides lots of opportunities for mutation. All of this makes it incredibly difficult for the immune system to fight them off.

Let’s take a look at a real-world example: HIV. HIV targets a certain kind of immune cell in the human body called CD4. These cells are also known as helper T cells. After breaking into the cell, HIV’s RNA and reverse transcriptase enzymes start creating lines of HIV DNA. That DNA makes its way into the CD4 cell’s nucleus, and sews itself into the cell’s DNA.

Now that it’s part of the cell’s genetic makeup, it can start wreaking havoc. Sometimes it can cause damage almost immediately. Other times, it can stay dormant for years while it’s reproduced all around the infected host's immune system.

The virus eventually disables the cells that it infects, or sometimes even kills them. Having a compromised immune system exposes the human body to all sorts of different dangers, and HIV eventually progresses into AIDS, a serious and often fatal condition. Fortunately, there are treatments for retroviruses.

In the case of HIV, the patient’s usually given a cocktail of drugs to fight the spread of the virus. These drugs usually do one of two things: prevent reverse transcriptase from becoming active, or stop HIV from entering the cell. But they can’t cure HIV — they can only slow it down.

Even though they’re so hard to fight, retroviruses might have helped shape human evolution — and we might even be able to use them to fight disease. By analyzing the human genome and comparing it to what we know about retroviral genomes, researchers estimate that somewhere between 1 and 8 percent of the human genome came from retroviruses. These viruses infected our ancestors millions of years ago, and the changes they made to their DNA stuck around.

These days, most of those virus genes have mutated to the point where they’re totally inactive and don’t really affect our lives at all. But a few have been linked to diseases, especially autoimmune diseases and some cancers. Hopefully, learning more about those links will help researchers develop new treatments.

And some researchers are looking into how to use retroviruses themselves as treatments. Since retroviruses can change a cell’s DNA, they can be used to insert new genes to fight diseases, like certain cancers. There have been some problems with these treatments, though, since retroviruses can be kind of unpredictable when it comes to changing a cell’s genome.

For example, they might insert the new gene right in the middle of another gene, which could interfere with important processes in the cell. But researchers are looking into ways to make retroviruses insert new genes only in very specific spots. So the ability that makes retroviruses so dangerous might also eventually make them a powerful treatment.

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