SWEENEY VIVIDLY remembers her kids petting the cows and riding joyfully in a wagon whose wheels were caked with mud and manure. What she didn’t know was that 26 cows on the farm were carrying 0157:H7, a particularly dangerous strain of the E. coli bacteria.

A week after the visit, 2-year-old Michael, who is autistic, came down with violent stomach cramps and was hospitalized. Not suspecting E. coli 0157:H7, which is rather rare, doctors treated him for stomach flu and sent him home. A few days later his urine turned dark brown, and Michael became pale and listless. His mother rushed him to the hospital, where he was diagnosed with kidney failure.

One-year-old Shannon, as it turned out, also was infected with E. coli 0157:H7, but did not become nearly as ill.

When Sweeney found out that the petting zoo and the local health department had known the cows might be infected but did not warn the public, she filed a class action lawsuit. In all, 16 children who had visited the petting zoo at Merrymead Farm in Worcester, Pa., contracted E. coli , and another 45 people were suspected to be ill from the bacteria.

While such outbreaks are rare, says Elichia A. Venso, director and associate professor of environmental health science at Salisbury State University in Maryland, the Merrymead Farm incident...

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