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In a typical year, San Diego County might see a few dozen cases of hepatitis A.

So far this year? More than 400, with 15 people now killed by the liver disease.

“This is an outbreak like none other that we’ve ever had,” said Dr. Wilma Wooten, the region’s public health officer.

Dr. Wooten said the response has been complicated by the infection’s nebulous spread. Whereas past outbreaks have commonly been traced to a single food source, allowing the threat to be swiftly contained, this one is passing person to person.

San Diego’s homeless population has been hit hardest by the virus, which stalks its victims more readily in areas of poor sanitation.