“I go right here,” said Linda, pointing between two knuckles. Her whole hand was swollen. “Can I hit it myself?”

Linda is a black 45-year-old heroin addict who’s been shooting up since she was 15. Scott is a white 35-year-old sociology professor on sabbatical from DePaul University. Last year he took phlebotomy training and learned to administer vaccinations and give shots to treat anaphylactic shock in preparation for a study on a hepatitis B vaccine, which is what led him to Linda.

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The hepatitis B virus attacks the liver. Symptoms include jaundice, fatigue, and vomiting. IV drug users are at high risk for the disease, which is transmitted through blood and other body fluids and can be spread by sharing needles, cookers, and cotton filters–60 to 80 percent of users have been infected. After recovering from an HBV infection, people usually have a natural immunity to the virus, but to curtail the spread of the disease, the Centers for Disease Control recommend vaccines for people in high-risk categories. Scott says the rate of vaccination for IV drug users and transient populations, who often “feel stigmatized and judged and tend not to trust traditional doctors,” is generally pretty low.

Scott says the data they collect will have implications not only for medical protocol but for public health policy. He interviews the participants during each visit and says he’s already collected “a shitload of data on needle use, social relationships, sexual behavior, hepatitis knowledge, and psychosocial indicators of health and well-being.” He thinks the information will be useful in related studies, including one he’s decided to do on overdosing. “Conventional wisdom is never shoot alone,” he says, “because nobody’s there to see you. But people who shoot alone tend not to overdose. They’re also less likely to contract communicable diseases.”

Scott dabbed the area she chose with an alcohol swab, but then Linda pressed down on the spot with her hand before going in with a new needle, pushing into her vein any germs she’d had on her fingers.

“Still dropping?” she eventually asked.

She said they were “burned.”