Spoilers ahead for War for the Planet of the Apes.

By the third film in the blockbuster Planet of the Apes series, it seems like the franchise should have wrung every last narrative drop from the virus that drove humans to near extinction and boosted apes’ brainpower. But in War of the Planet of the Apes, the simian flu flares up again, mutating to quietly spread through the last surviving people. Immunity against the original plague is no protection against the latest form of the virus, which causes a degenerative disease that leaves humans mute and animalistic.

That’s meant to bring this series in line with the original 1968 Planet of the Apes, where humans don’t speak — though it isn’t strictly necessary. The original implies that humans have devolved as a species, not succumbed to a silencing disease. Either way, the virus has to do a lot of heavy lifting in the trio of blockbuster films. It’s basically the magic bullet that explains every major deviation between this franchise and our familiar world.

As a result, the simian flu’s character arc is incredible, in the literal sense that it’s hard to believe. It’s clearly more fiction than science, but as scientists develop the technology to rebuild long-dead plagues from scratch, the simian flu deserves a closer look. Virologically speaking, could we actually see a real-world virus this contagious and adaptable? One that can cause such a massive variety of symptoms?

The short answer is no, says scientist Ann Powers, who studies emerging infectious diseases with the Centers for Disease Control and Prevention. “They’ve combined traits of several different families of viruses,” Powers says, creating an especially frightening, narratively useful “Frankenvirus.” But that’s not a bad thing, she adds, especially if it boosts interest in real-world viruses.

“It sounds like the directors and producers did whatever it took to make the virus produce a good storyline,” agrees Peter Hotez, dean of the National School of Tropical Medicine at Baylor College. And he can’t argue with the films’ knack for making virology look like a glamorous career path. In fact, had the series stuck more closely to reality, they might have ended up with another, less glamorous, franchise entirely, he says: Planet of the Sheep.

First, a little background: in 2011’s Rise of the Planet of the Apes, the simian flu starts out as ALZ-113, a virally delivered gene therapy for Alzheimer’s disease. Dr. Will Rodman (James Franco) designs a declawed retrovirus to insert the therapeutic genes into brain cells. (Retroviruses are the category of virus HIV belongs to, not the flu.) Rodman hopes to use the new, inhalant drug to save his father’s deteriorating mind, so he skips critical regulatory and safety steps. It goes horribly wrong when the virus turns out to be fatal to humans, and spreads across the globe.

Retroviruses don’t tend to be transmitted through airborne droplets, however, which is how the apes and humans are first exposed to the virus in the film. Instead, retroviruses typically require intimate contact, like sharing needles, unprotected sex, childbirth, or breastfeeding to hop from person to person. Hotez thought of one exception, though: a sheep retrovirus called Jaagsiekte that spreads via airborne droplets from sheep to sheep, causing lung cancer in the animals. Planet of the Sheep, he says, “would be the Weird Al Yankovic version” of the franchise.

Scientists really do use viruses as couriers to deliver foreign genes into cells, however, and retroviruses are especially good at this. That’s how retroviruses replicate in the wild: they slide their own genetic material into a host’s DNA, and hijack the host’s cellular protein-building machinery to churn out more virus particles.

When they’re used in the clinic for cancer immunotherapy, for example, parts of these viruses’ genomes are removed, so they can’t make copies of the original virus. Instead, they’re engineered to essentially become hollowed-out delivery vehicles that insert therapeutic genes, rather than their own, into a patient’s DNA. So it’s highly, highly unlikely that one of these declawed vectors might regain its viral traits and start multiplying.

Even so, research in this area is closely monitored for that remote possibility, says Stephen Russell, a Mayo Clinic scientist investigating virally delivered treatments for cancer. “The doomsday scenario you are looking at — i.e., the evolution of a virus-based therapy into a pathogen that destroys the human race — is associated with a vanishingly small probability,” he says in an email to The Verge.  

The viral plotline continues in War for the Planet of the Apes, when the virus mutates to spread among the survivors. This time around, simian flu 2.0 isn’t a killer — other humans are. Paranoid about the spread of this new plague, Woody Harrelson’s sadistic character, the Colonel, begins murdering the virus’s new victims. Another faction of humans fights back against his cruel militia, trusting that the planet’s remaining scientists can develop a cure.

Retroviruses are prone to mutating. When viruses make copies of themselves, it can be more like retyping a paragraph than copying and pasting it. Some viruses have a spellcheck function that keeps typos from sneaking in, but each time a retrovirus infects a new cell and starts worming its way into the host’s DNA, it has to type without spellcheck. That can introduce genetic typos, or mutations, that build up in the population of viruses circulating in the body, and in the community.

Many of these mutations won’t make a difference to the virus or its functions. Others will be harmful and the viruses harboring them will die out, says Powers’ CDC colleague, Ronald Rosenberg. “It’s a real crapshoot,” he says. But, the more mutations circulating, he says, the more likely it is that one might turn out to be advantageous if conditions change. Thats how adaptations like drug resistance can emerge.

Still, it would be an extraordinary feat for a virus to accrue enough mutations in just a few years to suddenly start rapidly spreading in a population that had previously been immune, let alone to produce such radically different, unrelated symptoms in victims. “Linking all of these things together is pretty close to being impossible,” Rosenberg says. “But I guess if you could imagine it, then it’s good enough for science fiction.”