"Unfortunately, recently, it hasn't been unusual at all," noted Rabbi Chaim Loike with the Orthodox Union, an organization that certifies whether products conform to Jewish dietary law.

Loikea says these worms were showing up in about one out of every six cans. He does not know why they have become so common, but he says it is not necessarily a new phenomenon.

"The Talmud, which was written 2,000 years ago, described a number of worms, which, even though they're not something you would want to eat, if they were accidentally consumed, would be kosher," Loikea explained.

The Talmud, a compilation of rabbinic opinions, debates and analyses, lays out the framework for Jewish law. It says if the worm comes from the intestines, it is generally not kosher. 

"However, if a worm is found to have grown its entire life in the flesh of the fish, it is considered to be the same as the fish," said Loikea. "And therefore, it's kosher." Intestinal worms might show up if the sardines are not handled properly.

But why would it matter if the worm is kosher? Most people would still find it disgusting. 

Well, if the rabbis decide that these worms, which have become so common in sardines, are not kosher, the Orthodox Union would no longer give the fish its seal of approval. That's a big deal, because even many non-Jews look for that certification as a sign of quality. Kosher foods are a $12.5 billion market.

"We're not advocating that people should eat worms. We're just researching whether or not we would have to de-certify all these things," Loikea added.

But Loike is a rabbi, not a parasitologist. He can't tell a gut worm from a flesh worm. So he went where anyone would go to find an expert: the Internet search engine, Google. 

"And we saw all the names of people who published papers on them and we started cold-calling them," said Loike.

Mark Siddall at the American Museum of Natural History in New York is one of the world's top experts on parasites. Siddall invited Loike to his office. The rabbi came with some cans of sardines, some tubs of fish eggs.

"[He also brought] a bag of previously frozen whole sardines, as well, that were dripping on the floor as we were walking to the elevators," recalled Siddall.

Dripping bags of fish aside, parasites do not disgust Siddall. He says they are all around us.

"We say, 'yuck' because we in Western society are kind-of like, 'Oh, parasites, that's horrible, right?' But it's actually quite normal for things to be parasitized," Siddall noted.

To figure out what kind of parasites Loike's fish had, Siddall used a technique called DNA barcoding. The genetic code of certain genes varies enough between species that researchers can use them to tell one from another. 

Siddall and his colleagues have used it to help the U.S. Fish and Wildlife service check for endangered species in smuggled goods. 

"There was a shipment of leeches that came in that were labeled as advertising material. And they were confiscated," Siddall said.

DNA barcoding identified them as endangered European medicinal leeches. Siddall had busted a leech-smuggling ring. 

When he DNA-barcoded Loike's sardine worms, he found five species. 

"And in all cases they were species we would normally expect [to find] in the muscle tissue or the ovarian tissue of the fish, and thus there was no indication whatsoever that there was improper handling," Siddall explained.

So the Orthodox Union issued a decision: the sardines remain kosher. 

Siddall says it's the first time DNA barcoding has been used for a strictly cultural issue. 

"And that's kind of cool, where you get cross-talk between science and culture," Siddall added.

As for Rabbi Loike, it strikes him that the parasites found in the fish are the same ones described in the Talmud. 

"The fish haven't really changed much in 2,000 years. They have the same parasites, the same everything. The world just keeps going and nothing's really changed," Loike noted.

We just use different tools to describe it.