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The U.S. Centers for Disease Control and Prevention (CDC) announced on Thursday that it is investigating three suspected cases E. coli infection related to the outbreak in Germany. All three people have recently traveled to Hamburg.

German health officials are still trying to trace back the origins of the E. coli infection that has killed more than a dozen people and sickened thousands of others, causing serious illness in 470. All are either German residents or have visited the country in recent weeks. They are focusing on raw lettuce, tomatoes and cucumbers as sources of the illness, particularly those consumed in northern Germany, and attempting to identify where these vegetables were grown.

(More on TIME.com: Why Health Authorities Are So Worried About Europe's Mutant E. Coli Outbreak)

To date, there have never been any confirmed cases of infection in the U.S. with the strain in question, E. coli O104:H4, although some cases have been reported globally in the past.

In general, says Dr. Brad Spellberg, a professor of medicine at Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center, E. coli packs a powerful one-two punch for those it infects. It belongs to a class of bacteria known as gram-negative bugs, which have a remarkable ability to swap genes with a wide range of other organisms. Mixing and matching genetic material can lead to dangerous combinations that allow E. coli to resist antibiotics or cause severe illness and even death in people.

What's more, E. coli are among the most common organisms in the human body, blanketing our gut in tremendous numbers, which makes the emergence of new, potentially harmful strains relatively easy. "The thing about infectious disease that we learned the hard way is that every time we catch up with something, something else mutates, and changes and evolves," Spellberg says. "It's the nature of the beast."

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What makes the currently circulating E. coli particularly worrisome is a combination of two factors. Through its game of genetic roulette, the strain has somehow acquired the ability to make a nasty form of toxin that it normally doesn't produce, known as shigatoxin. Although other strains of E. coli — including the O157 strain that contaminated hamburger meat and spinach in the U.S. in recent years and caused outbreaks of illness in the West and Midwest — have been known to produce this toxin, they have not produced this particular form.

In addition, O104:H4 has a powerful glue that allows it to stick to the gut wall, where it can more effectively release its toxin and harm tissues. "There have been reports of E. coli with this kind of 'glue,' which has mainly been an issue in children in the developing world, but in general they aren't associated with anything like the illness we've seen [in Germany]," says Dr. Robert Tauxe, deputy director of the division of food-borne, waterborne and environmental diseases at CDC.

Scientists at the Beijing Genomics Institute, working with German officials, have already sequenced the DNA of O104:H4 and have confirmed that it contains genes that make it adept at causing hemolytic-uremic syndrome (HUS), a serious illness associated with shigatoxin exposure. The condition can result in kidney damage or failure, requiring dialysis, or death. Already, 16 people infected with O104:H4 have died.

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The strain also contains genes that make it resistant to many commonly used antibiotics, but experts say that's less of a problem in this case, since it's not clear that antibiotics are the best way to treat these infections. For some reason, antibiotics actually enhance E. coli's ability to make shigatoxin, and some studies show that patients treated with the medications do worse than those who are provided supportive care. Such care can involve keeping patients hydrated with intravenous fluids and handling any kidney problems or consequences of strokes, which can occur when blood clots form as patients become anemic.

For now, the CDC is joining German authorities in warning Americans traveling to that country not to eat raw lettuce, tomatoes or cucumbers. Cooking contaminated food thoroughly also kills the bacteria in most cases, and washing hands frequently can lower the chances that any bacterial contamination is spread.

• Alice Park