The science of public health has evolved since the 18th century, when it focused on isolating the ill and quarantining those exposed to diseases to prevent transmission of infection. In the 19th century, the focus was on sanitation, especially separating fecal contamination from food and water. Living conditions improved tremendously as a result. With the understanding of infectious disease transmission in the early part of the 20th century, the focus shifted to immunizations and infection control.

Hansel Tookes, age 30, is an expert at identifying the detritus of injection drug use. As a public health student on hiatus from medical school, he led a group of researchers who walked the streets of Miami for four months, methodically counting discarded syringes in neighborhoods with high rates of drug arrests. As they crisscrossed more than 800 city blocks, the team spotted 328 used syringes, in parks, lots, and along sidewalks.