The rapidly developing European E. coli outbreak that has killed 19 people and sickened thousands, including four suspected cases in the United States, has become one of the deadliest outbreaks of E. coli in modern history.
Where exactly people are being infected with the disease is still unknown, although 17 people fell ill after eating in the northern German city of Luebeck in May, according to the local media. Researchers from Germany's national disease control center are inspecting the restaurant in question.
Other health experts suspect the disease first spread last month at a festival in the northern German city of Hamburg that was visited by 1.5 million people. But as of yet, there is no concrete proof that either site is the cause of the outbreak.
In a briefing Friday, the U.S. Centers for Disease Control and Prevention said the four suspected cases in the United States are all people who likely contracted the infection while in northern Germany in May and brought it back to the United States. Three of the victims are hospitalized with hemolytic-uremic syndrome and the fourth reported bloody diarrhea consistent with the outbreak strain of E. coli.
Two American military service members stationed in Germany are also suspected cases. The CDC said both of them have a similar diarrheal illness.
Government officials stressed that the outbreak has not affected the United States directly.
The Food and Drug Administration is monitoring lettuce, tomatoes and cucumbers from Spain and Germany based on information it has received from European investigators. Produce from those countries accounts for less than 0.2 percent of produce imported into the United States every year.
The FDA says it is also stepping up its food safety regulations.
Hygiene Key to Avoiding Spread
While the outbreak hasn't hit U.S. soil yet and the infection isn't easily spread, there are ways to be sure that any illness that may be caused by the outbreak isn't spread.
In addition to avoiding contaminated food, good hygiene is the most important way to minimize transmission, according to Dr. Maria Alcaide, an assistant professor of infectious diseases at the University of Miami's Miller School of Medicine.
"It's also spread through contaminated feces," Alcaide said. "Anyone who is sick should wash their hands, and their caregivers should as well."
Anyone who starts noticing any symptoms should get to a doctor.
"If anybody gets very bad, bloody diarrhea, they should get medical care immediately, and providers should also be aware of what's going on with the situation," Alcaide said.
ABC News' Kim Carollo contributed to this report.