Police said the worker switched sterile needles with contaminated ones, infecting at least 20 people, The Boston Globe reported Saturday.
No arrests have been made.
After weeks of investigating, the investigators said they think an infected worker stole sterile syringes containing drugs meant for the patients and replaced them with used syringes filled with water or saline.
More than 700 of the hospital's patients have been tested for the hepatitis C virus, the Globe said. Officials said former patients from as far back as back to Oct. 1, 2010, may have been exposed.
"People who are drug users -- they are not necessarily the cleanest people in the world," Dr. Jose Montero, New Hampshire's public health director, said.
Montero said the hepatitis virus survives on surfaces for a few days, so when investigators found infected patients who visited the hospital over several months and years, they started suspecting the source was one person.