It's what many food scientists feared: the E. coli strain responsible for an outbreak in Europe that has left 18 dead and sickened hundreds is a new hybrid strain capable of causing severe illness.

Scientists working for the World Health Organization (WHO) confirmed Thursday that genetic sequencing tests suggest the strain -- called E. coli O104:H4 -- is a never before seen combination of two different E. coli bacteria.

"This is a unique strain that has never been isolated from patients before," Hilde Kruse, a food safety expert at the World Health Organization, told The Associated Press.

Scientists at the Beijing Genomics Institute in southern China, who collaborated with colleagues in Germany on the testing, said the strain "is highly infectious and toxic." The bacteria is similar to a strain isolated in the Central African Republic known to cause serious diarrhea, the Chinese lab added.

WHO spokesperson Aphaluck Bhatiasevi added the agency is waiting for more information from other laboratories to learn more about these bacteria.

But Robert Tauxe, a foodborne disease expert at the U.S. Centers for Disease Control and Prevention, is questioning whether the strain is completely new and says it had previously caused a single case in Korea.

"This strain is rare enough that a lot of people haven't heard of it," he said

Researchers have been unable to find the food source of the illness. It has spread to at least 10 European nations and sparked fear about eating tomatoes, cucumbers and lettuce.

Many had suspected something strange was going on with the E. coli involved in this outbreak. It's already the third-largest involving E. coli outbreak in recent world history. And with 18 deaths so far, it may also be the deadliest.

Among the more than 1,500 people sickened, 470 have developed a rare kidney failure complication, called hemolytic-uremic syndrome (or HUS), and 18 have died, mostly in Germany, the country hit hardest by the outbreak.

Such a high rate of HUS as a proportion of total cases is unusual. Typically, HUS affects about 10 per cent of patients.

But since people with milder cases probably don't always seek medical help, it's difficult to estimate how big the outbreak is and therefore how virulent this new E. coli strain is.

What's also made this outbreak unusual is that many of those coming down with HUS have been adults, when the syndrome is typically affects more children than adults.

Hilde Kruse, a food safety expert at the WHO, told The Associated Press that there might be something particular about the bacteria strain that makes it more dangerous for adults.

She added that the new strain has "various characteristics that make it more virulent and toxin-producing" than the hundreds of E. coli strains that people naturally carry in their intestines.

It's not uncommon for bacteria to mutate and evolve by swapping genes. Strains of bacteria from both humans and animals can easily trade genes, similar to how viruses such as Ebola, influenza and SARS "learned" how to jump into humans.

But the question of where the illness is coming from continues to stump investigators. Reinhard Burger, head of German disease control agency the Robert Koch Institute, admitted Thursday there "still is no indication of a definable source."

Illness has even has caused some cases in the U.S., though those were among people who had recently travelled to Germany.

Although cucumbers were the first suspected culprit, officials now think it could be coming from lettuce, tomatoes and cucumbers.

Many also suspect it originated from organic vegetables, which use animal manure as fertilizer. E. coli bacteria typically lives in the intestines of cows and other ruminants and can spread through their feces.

To avoid foodborne illnesses, WHO recommends people wash their hands before eating or cooking food, separating raw and cooked meat from other foods, thoroughly cooking food, and washing fruits and vegetables, especially if eaten raw.

Experts also recommend peeling raw fruits and vegetables if possible.

With reports from the Associated Press