Public health officials tell the San Francisco Chronicle that it’s impossible to know how long E. coli 0157:H7 has been around. People likely were sickened by it for years, or even decades, before doctors identified it.

But the reason outbreaks have become more common in the past 25 years, health officials agree, is because technology has been developed to identify and connect strains of bacteria and because the nation’s eating habits have changed — we eat mass-processed foods that make it easier for contaminated products to reach more people.

Over the years, technology has become increasingly complex as federal health officials searched for ways to identify outbreaks more quickly. The technique used today, known as PulseNet, allows a microbiologist to track the "paternity" of a unique strain of 0157:H7, and, thereby, tell if isolated cases that appear around the country are connected.

The first E. coli outbreaks in the United States were in ground beef partly because E. coli bacteria live in cows, and partly because ground beef was among the first food products to be highly processed and mass-distributed via fast-food outlets.

In the 1990s, the source of the outbreaks spread to fruit and vegetables. In the past decade there have been 20 such outbreaks, including the most recent one. The last nine outbreaks involved leafy greens that were packaged into salad mixes.

The problem with those salad mixes is the same problem the meat industry ran into — a very small amount of contaminated vegetable can spread the E. coli bacteria to hundreds or thousands of packages when it’s mixed in a processing plant. That was the case with bagged spinach.

With meat, solving the problem meant simply cooking it at a high enough temperature to kill the bacteria. But raw vegetables may prove more challenging because there’s not a lot that can be done once the produce has been contaminated. Washing produce isn’t necessarily enough to get rid of E. coli.

For now, federal and state investigators are searching farms in the Salinas Valley for clues as to what caused the contamination in spinach. But they may never know the answer. And to some degree, bacteria are always going to be living in our food supply.