CSMP / Getty Images E. coli on a lettuce leaf

Health officials were worried enough about an unusually virulent outbreak of food-borne illness from the E. coli bacteria, which has infected more than 1,500 people in Germany and killed at least 17. But the concern jumped to another level on Thursday when the World Health Organization (WHO) announced that the responsible pathogen was a strain of E. coli that they believe had never before been seen by scientists.

(More on TIME.com: Q&A: A Food Safety Expert Explains Germany’s E. Coli Outbreak)

According to the Beijing Genomics Institute in China, which has been working with German scientists on the outbreak, the new strain is dangerous:

Bioinformatics analysis revealed that this E. coli is a new strain of bacteria that is highly infectious and toxic. This is a new serotype — not previously involved in any E. coli outbreaks. Comparative analysis showed that this bacterium has 93% sequence similarity with the EAEC 55989 E. coli strain, which was isolated in the Central African Republic and known to cause serious diarrhea. This new strain of E. coli, however, has also acquired specific sequences that appear to be similar to those involved in the pathogenicity of hemorrhagic colitis and hemolytic-uremic syndrome. The acquisition of these genes may have occurred through horizontal gene transfer. The analysis further showed that this deadly bacterium carries several antibiotic resistance genes, including resistance to aminoglycoside, macrolides and Beta-lactam antibiotics: all of which makes antibiotic treatment extremely difficult.

The preliminary genetic analysis indicates that the new strain is a mutant, the combination of two distinct groups of E. coli: enteroaggregative E. coli (EAEC) and enterohemorrhagic E. coli (EHEC). As is often the case when germs get together and start splicing, the result is not good for people, as Liverpool University biologist Dr. Paul Wigley told the BBC Thursday:

One nasty bacteria seems to have acquired a toxin from another nasty bacteria which has resulted in an even nastier bug. It seems it is producing two toxins which cause the damage and lead to bloody diarrhea and damage to tissues including the kidneys.

Indeed, the worst cases of the outbreak have involved acute kidney failure, which is often a life-threatening complication of normal E. coli outbreaks. But usually E. coli, like most food-borne illnesses, only poses a mortal threat to very young children or those who are already weak. In the case of the current outbreak, however, women make up more than two-thirds of those affected, and young and middle-aged adults — the very patients who should be able to weather the bacteria without major risk — form a very high percentage of the worst cases. Those acute cases are also occurring with unusual frequency; while kidney complications might occur in 5% to 10% of most E. coli outbreaks, Germany has reported 470 kidney failure cases out of about 1,500 known infections.

(More on TIME.com: German Egg Scare: Are Lax Food-Safety Laws to Blame?)

It’s important to remember, however, that we’re still in the early days of the outbreak, and it’s possible that there may be a number of much less severe cases that have simply gone undetected, which would lower the overall severity of the strain.

German doctors also use a broader definition than Americans for kidney failure. By the same token, the unusual age range of the victims could have less to do with the strain itself than the source, which might be a food children are less likely to eat. Still, scientists warn that the outbreak could continue for months.

Researchers are uncertain where the outbreak began, and they still don’t know what food might be carrying the contaminant. Reinhard Burger, the head of Germany’s Robert Koch Institute, initially pointed the finger at Spanish cucumbers, but that seems to be a mistake — albeit one that has cost Spanish farmers hundreds of millions of dollars.

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Indeed, produce farmers across the European Union are hurting, as consumers have stopped buying vegetables and fruits, afraid that anything might carry the pathogen. The situation may only worsen; Russia announced an immediate ban on all European fresh produce in response to the outbreak. Although the E.U. has protested, if Russia holds up its ban, it will hurt: the Russian market for European produce is worth $5.5 billion a year.

Moscow’s move is almost certainly an overreaction, but it’s not an unusual one in these kinds of food-borne illness outbreaks, especially one that looks to be this severe. The outbreak has spread beyond Germany to several other European countries, with three people infected in Britain. Americans who’ve traveled to Germany recently have been told to watch for symptoms.

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Disease detectives will work hard to trace the outbreak back to its source, but given the complexity of the European produce market — and the comparative lack of a unified pathogen databank of the sort kept in the U.S. — the original culprit may never be discovered. This mystery is one that isn’t likely to end soon.

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