America tries to eat its vegetables, so the recent romaine lettuce E. coli outbreak has put everyone from Caesar salad fans to those who barely tolerate a splash of greens in a taco on high alert.

But how did these seemingly innocent little leaves turn hazardous?

The Centers for Disease Control and Prevention has tallied 40 cases in 16 states stemming from the romaine lettuce grown in the Salinas, California, region, including 28 hospitalizations.

We've sent men to the moon. Many of us carry tiny computers in our pockets. So why can't we keep romaine lettuce safe and clean? Here's some reasons why lettuce is so vulnerable and why tracking down the source of a bacterial outbreak can be difficult.

>>E. coli outbreak alert: Do not eat any romaine lettuce from Salinas, California, CDC says

America hearts lettuce, especially women

Thanks to the national wellness trend embraced by healthy millennials and aging Baby Boomers, the consumption of fresh vegetables, as opposed to frozen or shelf-stable varieties, is on the rise. Adding to that is the growing popularity of salad-centric restaurants, like Tender Greens and Sweetgreen, and the increasing inclusion of salads on the menus of fast-food chains.

According to global market research firm Mintel's data, 70% of vegetables sold in the U.S. in 2016 were fresh produce, up 13% from 2011, but growing at a 39% clip since then is fresh-cut salad, now 11% of what's in stores. Fresh veggies are forecast to grow 9% by 2021 and fresh-cut salads, 33%.

The group most affected by the romaine lettuce E. coli outbreak is women. According to the CDC, 65% of the people sickened as a result of this episode are female. Chalk that up to women being bigger salad eaters than men.

>>Food recalls: Feel like there's a food recall every day? That shouldn't stop you from eating

Blame it on Mother Nature

Fields where produce is grown are subject to the whims of Mother Nature and her animals. Fruits and veggies grow in dirt and can be fertilized with manure. Bugs and birds fly around. Animals may run wild through even fenced fields – or defecate in rivers and lakes used to irrigate nearby farms. Growing out in the open means lots of opportunities for bacteria to enter the picture.

"Stuff is grown in nature outside. It can't be bacteria-free, virus-free, parasite-free," University of Florida food-safety expert Keith Schneider said. "We do our best. The farmers jump through a lot of expensive hoops to make the food as clean as possible, but nature can intervene."

Lettuce is naturally unprotected

Unlike some of its fruit and vegetable brethren, lettuce has nothing to keep it safe. The lack of rinds and peels – which you'd find on, say, a watermelon and a cucumber – gives bacteria countless entry points. A head of lettuce has tons of nooks and crannies for the nasty stuff to hide, making leafy greens far more vulnerable than other thinly-skinned, yet solid and easily washed produce, like tomatoes.

"The population is choosing to eat foods inherently more risky," said Matthew Stasiewicz, an assistant professor of applied food safety at the University of Illinois at Urbana-Champaign. "Realistically, there’s not an activity in life that carries zero risk."

Distance can make the problem grow faster

The days of getting produce year-round only from nearby farms are long gone. As the national food supply becomes more sophisticated, we're able to grow food at a central location and then pack it, preserve it and ship it around the U.S.

Despite the ease social media and 24-hour news cycles bring to spreading warnings about outbreaks, following the food from point A to point E remains complicated.

Convenience can lead to illness

The extra phase of processing the romaine lettuce – whether chopping it or simply packaging it – provides more chances for bacteria to sneak in.

"If all lettuce from the field is clean, you bring it to the packing house, and equipment is contaminated," Schneider said. "All things that go over the belt, we can contaminate."

A raw deal

Foods you eat raw, like lettuce, miss out on a cooking phase that can kill bacteria. What the industry has dubbed "the kill step" withholds a key opportunity to get rid of food poisoning-causing particles.

For example, applesauce, which is subjected to heat as part of the preparation, is far less likely to pose a problem than a raw Red Delicious apple. That's why health officials preach the importance of thoroughly cooking meat and poultry and to avoid unpasteurized dairy products. For produce, washing is key.

"One of the major challenges of identifying sources of foodborne disease is people eat a lot of food," Stasiewicz said. "There’s also a major problem remembering what they eat. When eating a complex food, like a sandwich or burrito, you might not even know what you're eating."

The evidence is often tossed

Fresh produce has a short shelf life, especially quickly-spoiling bagged salads, so by the time the inspector made it to your house, the culprit would be long since tossed.

While you know what your favorite breakfast cereal is, chances are you don't recall the name of the company that makes your bagged lettuce. Forget about remembering the brand of lettuce heads or hearts you buy. That complicates tracking.

"Without a bag, a lot number to a farm, you don't know where it was packed and what farm it came from," Schneider said.

And even if you miraculously still had the tainted lettuce, complete with the packer's or farm's name printed on it, the crop in the field by the time inspectors arrived would already be harvested.

Solving the problem isn’t simply a matter of “lettuce CSI," according to Schneider. "When you want to find the pathogen smoking gun, it’s gone."

Editor's note: A version of this story was originally published in May 2018 during an E. coli outbreak stemming from the romaine lettuce grown in the Yuma, Arizona, region.