In May 2012 a hospital in small-town Exeter, N.H., notified the state of a possible cluster of linked hepatitis C cases. Four people had recently been diagnosed, and testing soon revealed that the genetic codes of the strains that they carried were nearly identical. Because hepatitis C virus (HCV) mutates rapidly, epidemiologist Jose Montero knew that the infections most likely originated from the same person. “We knew we needed to find this person immediately,” says Montero, the state's director of public health.

The four also shared a common history—three had been patients at Exeter Hospital's Cardiac Catheterization Laboratory, and one worked there. As the investigators began testing former laboratory patients, they identified 29 more cases. By sequencing the genomes of the viruses in the outbreak, Montero's colleagues at the state's Public Health Laboratories constructed an evolutionary tree that led back to lab employee David Kwiatkowski. Police believe Kwiatkowski injected himself with the clinic's narcotics and reused the needles on patients.

More and more, scientists are helping to solve crimes using molecular techniques originally designed for epidemiology—tracking the spread of disease through large populations. In a similar application of forensic epidemiology, Fernando González Candelas of the University of Valencia helped to retrace an even larger outbreak in Spain, where an anesthesiologist was suspected of spreading HCV to hundreds of patients.

Candelas's group compared the genomes of viruses contracted in the outbreak with those of other HCV strains circulating in Valencia. The researchers found that people sickened in the outbreak carried a virus that was significantly closer to the anesthesiologist's strain of HCV than anything going around in the community, indicating that the doctor had almost certainly infected them. Candelas and his colleagues were also able to estimate when a person became infected to confirm that the infection occurred while that person was under the doctor's care.

The researchers recently published in BMC Biology the details of their inquiry, which in 2007 helped to convict the anesthesiologist of infecting 275 people. “People can live with HCV for a long time and infect large numbers of people,” Candelas says. “The only way we can prove that they are the source of the infections is through forensic epidemiology.”

Despite the widening use of epidemiology in the courtroom, it has its limitations. In one critique, two researchers noted that viral reconstructions involve a degree of interpretation and estimation. They cautioned that jurors may place unwarranted confidence in the science “and fail to comprehend or fully engage with its complexities and inherent shortcomings.”

The New Hampshire hepatitis case never reached a jury. After Montero worked with the fbi to build a criminal case that uncovered a trail of HCV infections and drug infractions across several states, Kwiatkowski pled guilty in August. Although Montero cannot say with certainty why Kwiatkowski accepted a plea deal, “I'd like to believe it was on the strength of our epidemiological investigation.”