The Centers for Disease Control is investigating an outbreak of 17 E. coli infections that were reported in 13 states across the United States. Other cases have also been reported in Canada and the Public Health Agency of Canada is looking into it as well.

The cases were all occurred between Nov. 15 through Dec. 8, 2017. Genome sequencing is being done by the CDC to determine whether the same thing is making people sick in the U.S. as it is in Canada. Early results show that the bacteria are very similar in nature meaning the source of the bacteria is probably the same for cases in both countries.

Canada has identified romaine lettuce as the root cause of the illnesses there. The CDC is conducting interviews with those who are sick to determine whether they all consumed the same item, like romaine or other leafy greens, in the week before their illness struck.

“Because we have not identified a source of the infections, CDC is unable to recommend whether U.S. residents should avoid a particular food. This investigation is ongoing, and more information will be released as it becomes available,” the CDC said.

What is E. coli?

Escherichia coli is a bacteria that naturally occurs in the environment as well as in the guts of animals and humans. Most strains of it are harmless but a select few can make people incredibly sick. Some strains of the bacteria can cause diarrhea, but other can cause more severe illnesses like urinary tract infections, respiratory illness and pneumonia.

What are the symptoms?

Symptoms really vary from person to person who is infected with the bacteria and will usually appear three to four days after the bacteria is ingested, although they can appear anywhere between one and 10 days after exposure. Many people experience diarrhea, vomiting, and strong stomach cramps, according to the CDC. Some people also experience a slight fever or blood in the stool. Most people will experience symptoms for five to seven days and the severity of the illness can vary significantly. Anyone who has a hard time keeping liquids down or is passing very little urine should contact their doctor.

An e. coli outbreak might be caused by contaminated romaine lettuce.