BERLIN — An unusually lethal strain of E. coli bacteria has infected more than 1,500 people in Germany, mystifying public health officials, ravaging Spain’s agricultural heartland, and touching off panic in Europe as people weighed whether it was safe to eat raw vegetables.

The source of the outbreak, which has killed at least 16 people — 15 in Germany and a Swede who visited there recently — remained unknown.

Public health officials are alarmed because a startlingly high proportion of those infected suffer from a potentially lethal complication attacking the kidneys, called hemolytic uremic syndrome, which can provoke comas, seizures and stroke. Dr. Robert Tauxe, deputy director of food-borne disease at the Centers for Disease Control and Prevention in Atlanta, said the rate of cases of acute kidney failure in the outbreak was unprecedented. “That makes this an extraordinarily large and severe event,” he said.

While most of the infections were among people who had traveled to northern Germany, the authorities acknowledged that the outbreak had spread to virtually every corner of the country.