In the last 24 hours, it is fair to say that tons of romaine lettuce has been removed from America’s store shelves. Another E. coli O157:H7 outbreak linked to romaine lettuce probably has you wondering wher this contaminated product is coming from and how is it related to past outbreaks.

But first, let’s review. On Tuesday, we learned of this new outbreak that has infected 32 people in the United States and another 18 in Canada.

In the U.S., the 32  people infected with the outbreak strain of Shiga toxin-producing E. coli O157:H7 live in 11 states. Their illnesses started on dates ranging from Oct. 8 to Oct. 31. Thirteen people have been hospitalized, including one person who developed hemolytic uremic syndrome, a type of kidney failure.

In Canada, 18 people sickened with the same DNA fingerprint of E. coli O157:H7 bacteria as the U.S. patients are from two Canadian provinces: ontario and Quebec. The 50-person outbreak has not resulted in any deaths on either side of the international border.

But, epidemiologic evidence from the United States and Canada indicates that romaine lettuce is a likely source of the outbreak.

And wher was the contaminated romaine grown? Nothing definitive yet, but the early betting is that it came from California’s Monterey County or the surrounding growing season, wher the romaine lettuce growing season is in full swing.

Monterey County Agricultural Commissioner Henry Gonzales told local media it is “very likely” the poisoned romaine came from his country. At the center of the Salinas Valley, Monterey County last year produced more than 40 million cartons — 24 heads to a carton — of romaine lettuce. That’s about half of the U.S. total annual production.

While we wait for the formal investigation to officially name the growing area, it’s important to note that this is the third E. coli outbreak in the past year to involve romaine lettuce or leafy greens. Just to review, the two prior outbreaks before this current episode were:

Dec. 12, 2017 — Twenty-five illnesses reported in 15 U.S. states with nine hospitalized and one dead. Whole genome sequencing showed that the E. coli strains that made people sick in Canada and in the United States were closely related genetically. Romaine lettuce in Canada, and leafy greens in United States were identified as culprits. Both from unidentified sources.

April 10, 2018 — First announcement issued on a second E. coli outbreak involving romaine lettuce, with 210 confirmed cases in 36 states, causing 96 hospitalizations and 5 deaths.  The contamination was traced to the romaine lettuce from multiple farms in the Yuma growing region, with contaminated canal water found to be a possible common denominator.

Ill people in this new outbreak, announced on Nov. 20, were infected with E. coli bacteria with the same DNA fingerprint as the E. coli strain isolated from ill people in a 2017 outbreak linked to leafy greens in the United States and to romaine lettuce in Canada.

The current outbreak has not been linked to the immediately previous multi-state outbreak of E. coliO157:H7 infections linked to romaine lettuce. The outbreak strain from this past spring’s outbreak does not match current patients.

In the latest announcement, CDC on Nov. 20 advised that consumers should not eat any romaine lettuce because no common grower, supplier, distributor, or brand of romaine lettuce has been identified.

In 2017, the CDC, several states, and the U.S. Food and Drug Administration investigated a multistate outbreak of Shiga toxin-producing Escherichia coli O157:H7 (STEC O157:H7) infections.

Twenty-five people infected with the outbreak strain of STEC O157:H7 were reported from 15 states.

Illnesses started on dates ranging from Nov. 5 to Dec. 12, 2017.

Nine people were hospitalized, including two people who developed hemolytic uremic syndrome, a type of kidney failure. One death was reported from California.

In December 2017, the Public Health Agency of Canada (PHAC) investigated the outbreak of STEC O157:H7 infections in several provinces linked to romaine lettuce.