David Kwiatkowski was a traveling medical technician and authorities say he's had Hepatitis C, and a drug habit, for at least two years.
In New Hampshire he's charged with stealing drugs in a syringe.
He allegedly injected himself, refilled the syringes with saline and returned the contaminated syringes, which were then used on patients.
"If he knew he was infected, and he put those needles back on the shelf, I mean, that is a definition of evil," said ABC medical editor Dr. Richard Besser.
The fear is that Kwiatkowski may have committed the same crime elsewhere.
In the past five years he has worked in 13 hospitals in eight states.
That means tens of thousands of people are potentially at risk.