The E. Coli outbreak that has sickened 1,000 and killed 16 so far in northern Germany involves one of the rarest strains of the disease known to science. Originally reported to be from Spanish cucumbers, German authorities have now admitted their suspicions were wrong and they don't know where the latest outbreak originated. But what they do know is that the strain of the disease that threatens northern Germany and greater Europe is a particularly rare and nasty one known as E. Coli O104:H21. 

The more common strain identified as E. Coli O157 affects some 70,000 people in the United States each year, causing bloody diarrhea, abdominal cramping, and sometimes death. But its treatment and tracking is more familiar to scientists than the more aggressive and rare strain currently affecting Germany. In fact, the O104:H21 strain is so rare that leading German E. coli researcher Helge Karch had in 30 years only heard of one other outbreak of the strain, according to Der Spiegel.

On Tuesday, the German newspaper Süddeutsche Zeitung reported that Karch had discovered that the O104:H4 bacteria responsible for the current outbreak is a so-called chimera that contains genetic materia from various E. coli bacteria. It also contains DNA sequences from plague bacteria, which makes it particularly pathogenic. There is no risk, however, that it could cause a form of plague, Karch emphasized in remarks to the newspaper.

After determining the strain of the bacteria, the race was on for scientists to determine its source. Late last week, they thought they had narrowed it down to cucumbers from Spain, but then Hamburg health minister Cornelia Prüfer-Storcks announced today that Spain had been ruled out as the source. E. coli live in the lower intestines of mammals, which means food contaminated with the bacteria likely came in contact with feces (such as the fertilizer sometimes used to grow produce). But farmers only use that fertilizer before sowing their seeds, so researchers are turning their attention elsewhere:

Now the scientists are taking a closer look at a group of pests that were previously above suspicion: slugs. Biologists from the University of Aberdeen in Scotland identified the mollusks as potential E. coli carriers -- the bacteria can survive for up to 14 days on the slimy surface of their bodies. Arion vulgaris, the Spanish slug, has long been a problem in Germany, but it's an even bigger and more widespread problem in its native Spain.

The outbreak has already been called the world's largest, and without a cause identified it's likely to increase. Health officials are warning people in Europe to take special care to wash their hands frequently and cook vegetables for the time being.