It is hard to conceptualize all the fast-food menu items that did not exist a decade ago.

There was no KFC Double Down. Starbucks' Unicorn Frappuccinos had not yet graced Instagram — which did not even exist. And, 2019's Chicken Sandwich Wars were unfathomable.

However, no menu item changed fast food in the last decade as much as Taco Bell's Doritos Locos Taco.

The chain sold more than a billion Doritos Locos Tacos when it launched in 2012, hiring an estimated 15,000 workers to keep up with demand.

"The idea sounds really simple, but it has to deliver on two fronts: the classic Taco Bell taste and the distinctive Doritos experience," Taco Bell product developer Steven Gomez told Business Insider. "Unlike a tortilla chip, taco shells can't break, and have to properly hold the taco ingredients."

Doritos Locos Tacos helped kick off an era of fast food in which social media became a crucial element in everything from the creation of new menu items to marketing.

The idea of Doritos Locos Tacos gained much of its momentum after Todd Mills started a Facebook movement around the idea in 2009. The group eventually gained thousands of followers, and Taco Bell flew Mills to a test kitchen to try the product for the first time in 2012.

In the years since, fast-food chains have constantly attempted to recapture the online buzz of Doritos Locos Tacos. However, few menu launches have the staying power of the Doritos Locos Taco, which remains a customer favorite.

Doritos Locos Tacos also sparked a massive change at Taco Bell.

Prior to the launch of Doritos Locos Tacos, Taco Bell was struggling. Doritos Locos Tacos were the chain's biggest launch of all time, fueling a much needed turnaround. Today, Taco Bell is parent company Yum Brand's crown jewel, with 2018 marking the chain's seventh consecutive year of same-store sales growth.

Brian Niccol was Taco Bell's chief marketing officer when the chain launched Doritos Locos Tacos. Niccol became Taco Bell's CEO in 2014. And, in 2018, Chipotle announced it had tapped Niccol to recreate Taco Bell's Doritos Locos-fueled comeback magic at the rival chain.