“Retrovirus: when it comes back, you don’t really want to be there.”

“Retro” is in right now.

Of course, retro is always in. In the ’70s, people pined for the ’50s. In the ’90s, they wanted the ’70s back. And now, it’s ’90s nostalgia. So is a “retrovirus” just a cold bug that dresses like Blossom and listens to Nirvana CDs?

No. For the love of everything holy in this world, it is not.

A retrovirus is instead a virus that uses a process called reverse transcription. Because retroviruses — like Blossom, but without the goofy hat — just had to be different.

Nearly every organism on the planet follows what biologists call the “central dogma”. That’s the rule that says genes coded in DNA get converted to RNA, and that RNA is then read to make proteins, which are the building blocks for cells, people, animals, plants, Joey Lawrence and the cotton inside grungy flannel shirts. Among other things.

That’s the way life works — DNA to RNA, in a process called transcription, and RNA to proteins, which is called translation. It’s a solid system, and everybody follows the same rules.

That includes most viruses, who are little more than a few scraps of DNA and maybe a protein shell to hold it all together. These viruses infect cells, get their DNA converted to RNA by the cell’s machinery, then to protein, package themselves up and look for the next cell to invade.

Nobody ever said viruses live fulfilling lives. They’re like an old retired couple with an RV, wandering aimlessly in search of early bird dinners and cheap campground fees. There’s no point, exactly, but it keeps them busy.

And in the virus’ case, it also keeps them causing flu, smallpox, herpes, warts and sometimes cancer. It’s not a perfect analogy. Old people aren’t quite as harmful as all of that. Mostly.

Retroviruses, though, refuse to play by the rules. A retrovirus doesn’t pack its DNA on road trips; it bundles up RNA instead. It also packs a special type of enzyme called reverse transcriptase. This protein flips the central dogma upside down, and can make DNA out of the retroviral RNA. This new DNA then worms itself into the host genome, where it gets converted to RNA and protein, as usual.

So retroviruses aren’t so much like the retired RV couple scoping out campsites. They’re more like a biker gang that invades your neighborhood, squats in your house and drinks from all your toilets. And not in the nice way.

Because they randomly insinuate themselves into chromosomes, retrovirus DNA can sometimes cause cancer by disrupting an important gene. And that’s on top of the diseases they cause to begin with, which include AIDS and related diseases, equine infectious anemia, avian wasting disease, encephalitis in sheep and goats, and several others.

Not all retroviral infections are harmful, though — or even active. Sometimes, a retrovirus inserts its DNA into a “silent” stretch of DNA and it’s never heard from or activated again. Like Ugly Kid Joe and Starter jackets. These “endogenous” retroviruses are so common, in fact, it’s thought their sequence makes up 5-8% of the human genome.

So when it comes to retroviruses, they’re much like “retro” trends: better left buried and forgotten than dug up, reawakened and unleashed on anyone or anything you care about. And if a retrovirus should get loose? Hide the Blossom hats and Nirvana CDs; you’re in for a rough ride.