Hollywood is fighting off a nasty illness.

“Sequelitis,” the entertainment industry equivalent of the Zika virus, has gripped major studios. Its symptoms include sluggish box office, feverish critical take downs and disdainful social media reactions.

At least, that’s what analysts and executives are telling themselves after “Teenage Mutant Ninja Turtles: Out of the Shadows” became the latest sequel to disappoint. It follows a long line of follow-ups and spinoffs that flopped or failed to live up to their predecessors — a group of underachievers that includes “X-Men: Apocalypse,” “Alice Through the Looking Glass” and “Neighbors 2: Sorority Rising.”

“Audiences are challenging us to make excellent movies,” said Rob Moore, vice chairman of Paramount Pictures, the studio behind “Teenage Mutant Ninja Turtles.” “The fact that it’s part of a franchise or sequel doesn’t let you off the hook. You need to raise the bar and make the story exciting, compelling and fun.”

If it’s true that audiences are rejecting franchises en masse, it undermines the financial underpinnings of the movie business and it means that major studios should brace for a punishing summer. After all, it’s not like the sequels are showing any signs of stopping. Over the coming weeks, moviegoers will be treated to fresh installments in the “Ghostbusters,” “Independence Day,” “Finding Nemo,” “Star Trek,” “Jason Bourne” and “Ice Age” series. In fact, nearly every weekend this summer will offer up at least one sequel, reboot or spinoff.

There’s a financial reason for the pile-up. Franchises are the straw that stirs a studio’s drink. As the domestic theatrical business slows, the one area of growth is the foreign box office. To that end, sequels tend to travel, playing particularly well in markets such as China that have become a critical source of revenue.

Franchise films also lead to greater merchandising opportunities. Characters from sequels are more likely to pop up in ads for cars or fast food. They’re the inspiration for toy lines, t-shirts and theme park rides. Studios are small parts of sprawling media conglomerates. Their value isn’t measured in box office. They exist to create intellectual property that can pollinate the consumer products, television and other divisions of the Comcasts and Time Warners of the world.

Franchise mania isn’t new. But it has intensified after the success that Disney enjoyed with its Marvel films. The interlocking superhero stories featuring the likes of Iron Man, Captain America, Thor and other costumed Avengers, have created a thirst for cinematic universe building. Every studio wants to be in the game. Even though the reception to “Batman v Superman: Dawn of Justice” was mixed, Warner Bros. is plunging ahead with a half-dozen films based on DC Comics heroes. Its next installment, “Suicide Squad,” debuts in August. Not to be outdone, Universal has begun producing a series of films based on monsters such as the Mummy, the Invisible Man and Dracula that will see various creatures and freaks of nature interacting over the course of several sequels and standalone adventures. Given that Disney is the envy of its major studio rivals, it’s understandable it would inspire imitators. There’s a danger, of course. As studios go farther and farther down the intellectual property food chain, the cinematic universe construction boom could eventually bust.

Despite the sequel swan dive, it’s too early to declare that the movie business is living through a bubble. Many of the sequels that sputtered at the multiplexes have been artistically inferior. The latest “Teenage Mutant Ninja Turtles” only received an anemic 37% “rotten” rating on Rotten Tomatoes, an improvement on the 22% rating for the first film, but hardly enough to guarantee a slot in the Criterion Collection. Other recent sequels fell short in the review department, with “Alice Through the Looking Glass,” “Zoolander 2,” “Ride Along 2,” “The Huntsman: Winter’s War,” “X-Men: Apocalypse” and “The Divergent Series: Insurgent” all suffering worse notices than the films that preceded them.

It’s not like audiences have rediscovered their love for movies without roman numerals affixed to them. “The Nice Guys” and “Popstar: Never Stop Never Stopping” whiffed at the box office, while upcoming films such as “Warcraft,” “Tarzan” and “The BFG” face uphill battles this summer. They serve as a reminder of the risks that studios take when they try to launch original movies in the height of popcorn season. Original, itself, has become a neutered term, given that “Warcraft” is based on a popular video game and Tarzan has been swinging through the jungle in various incarnations since the dawn of movies.

Some of the problem with the current crop of sequels is that they are rush jobs, argues Jeff Bock, an analyst with Exhibitor Relations. Two years separated installments in the “Neighbors,” “X-Men,” “Teenage Mutant Ninja Turtles,” “God’s Not Dead” and “Ride Along” series. That’s not enough of an absence for the heart to grow fonder, Bock believes.

“The incubation period needs to be longer,” he said. “Hollywood is not good at being patient, but maybe it needs to learn to wait a little longer, because these films are suffering under the weight of not being given enough time. “

Waiting certainly helped “Star Wars: The Force Awakens” and “Jurassic World,” franchises that took a decade-long break and re-emerged stronger than ever. In those cases, parents who were weaned on tourist-eating dinosaurs and galactic warriors had come of age in the ensuing years. They wanted to share a formative moviegoing experience with their own children, creating hits that spanned generations.

That’s good news for “Ghostbusters” and “Independence Day: Resurgence,” both of which revive franchises that have been dormant for 20 years or more. Not everything benefits from a longer gestation, of course. Ben Stiller waited 15 years before reviving Derek Zoolander on screen, but critics feasted on “Zoolander 2” and the film collapsed at the box office. It doesn’t matter how beloved the first film in a franchise was, part two or three or six still needs to be good.

“A sequel rises or falls on its own merits,” said Paul Dergarabedian, senior media analyst at comScore. “If a movie doesn’t deliver, it’s not going to create the same level of excitement and interest.”

Sequels that have outperformed previous film(s), since January 2015:

Star Wars: The Force Awakens

Jurassic World

Captain America: Civil War

Furious 7

Pitch Perfect 2

Minions

Hotel Transylvania 2

Batman v Superman: Dawn of Justice

Mad Max: Fury Road

Sequels that fell short of previous film in series:

X-Men: Apocalypse

Alice Through the Looking Glass

Spectre

Kung Fu Panda 3

The Divergent films

The Hunger Games films

London Has Fallen

Magic Mike XXL

Zoolander 2

Paul Blart: Mall Cop 2

God’s Not Dead 2

My Big Fat Greek Wedding 2

Neighbors 2: Sorority Rising

Barbershop: The Next Cut

The Huntsman: Winter’s War

Ride Along 2

Ted 2

The Woman in Black 2

Mission Impossible: Rogue Nation

Avengers: Age of Ultron

Insidious: Chapter 3

Sinister 2

Taken 3

Alvin and the Chipmunks: The Road Chip

Maze Runner: The Scorch Trials