Federal health officials were uncertain as to whether the bacterial strain that contaminated spinach and infected people coast to coast is unusually virulent, but more than half the reported cases have required hospitalization.

A key unanswered question is whether the strain of E. coli O157:H7 is particularly virulent, reports Newsday writer Delthia Ricks. At this point all data would point in that direction given that 66 people, slightly more than half of the 131 confirmed cases were hospitalized, Acheson said. In a typical outbreak involving E. coli O157:H7, he said, health experts would expect 20 percent to 25 percent of people to be hospitalized. Hospitalizations occur because patients develope hemolytic uremic syndrome, the potentially fatal kidney disorder caused by the bacteria’s toxin.

Dr. Ken Lee, director of the Center for Food Safety at Ohio State University, said in an interview Tuesday that "no one should risk dying from eating spinach." Among microbial culprits that routinely attack foods, he said, "E. coli is not the biggest killer in terms of body counts." But it is particularly insidious because it can cause bloody diarrhea and hemolytic uremic syndrome, he added. "We need to do a better job of cleaning up food."

Health investigators are examining practices on nine farms, but the probe could expand. Product recalls could involve other companies, but federal health officials hope spinach growers outside of California will soon be able to market their products.