The same strain of bacteria was found in a sample at Whittier Dairy, a bottle of milk found in a victim's home, and the blood of four people diagnosed with the infection, The Boston Globe reported. The bacterium was found in one sample at the dairy, while others tested negative.

"The pattern is very unique," Dr. Alfred DeMaria, director of communicable disease control, said. "It means there's an outbreak here. There's no question there's an outbreak. And it implies that the dairy is the common source."

The 19 other cases of Listeriosis found in the state in 2007 and 99 since 2002 all had different genetic fingerprints.

Whittier, which has its headquarters in Shrewsbury, is known as an old-fashioned milk producer that still makes home deliveries. The state said the company's pasteurization procedures appear excellent, suggesting that contamination occurred during bottling.