Information collected to date indicates that romaine lettuce from the Yuma, Arizona growing region could be contaminated with E. coli O157:H7 and could make people sick.
Wildflower Bread Company, an Arizona restaurant chain, said all of its romaine lettuce comes from California, which has no connection to the outbreak.
In the weeks since then, that total has grown to 84 individual cases reported, stretching across 19 states.
The CDC reports 42 people out of the almost 90 cases so far have been hospitalized from E. coli.
The CDC confirmed to Newsweek that this outbreak is the worst multi-state E. coli outbreak in the US since 2006, when almost 200 people got sick from contaminated spinach. More than 40 people have gone to the hospital, including nine with HUS-however, no deaths have been reported.
Health officials said that most people develop symptoms like diarrhea, severe stomach cramps, and vomiting - and recover within a week.
"The next step would be to work together, again this is in Arizona, with our Arizona counterparts, other trade associations, regulatory agencies to try and try to find the root cause and decide if there is something that can be done to ensure this doesn't happen again", he said. Eight of those 12 have been hospitalized, and one patient developed HUS.
If you don't know where the lettuce is from, the CDC says, don't eat it.
The CDC warning continued: "Product labels often do not identify growing regions; so, throw out any romaine lettuce if you're uncertain about where it was grown". However, the CDC has also shown that leafy vegetables were responsible for 22 percent of foodborne illnesses between 1998 and 2008.
Tekac said consumers should abstain from purchasing any romaine lettuce or products containing romaine lettuce grown in the Yuma region.
"This bacteria can actually get inside the lettuce leaf", he said.
This investigation is ongoing, and CDC will provide more information as it becomes available.