"No hot chocolate today?" she asks.

The volunteers, who generally bring hot chocolate or sandwiches, shake their heads and promise to bring the coveted drink the next day. After a few more minutes of chatting, the teenagers leave the van, disappearing into the cold night without any mention of matters medical.

The van, separated into three separate spaces, is set up to allow such informal visits. The area where the hot chocolate is usually kept is designed for casual conversation. On one side is a brightly-colored collage of pamphlets, running the gamut from information about job opportunities to sexually transmitted diseases to a "street survival guide," listing places to find free meals or clothing. A large poster on one wall urges teenagers to "exercise your right to be healthy."

Van rules handwritten in red magic marker are tacked on the wall. The rules prohibit alcohol, drugs and violence in the van.

These rules are respected. The van is a police-free zone, meaning that the Cambridge police have agreed not to come into the van unless they are required to by an emergency.

"This helps the kids know that it's a place that belongs to them, so they need to respect it," says Maurice W. Melchiono, a nurse practitioner who volunteers with the medical van. "It's important for us to be able to foster a sense of safety for the teenagers."

A gray curtain separates the drop-in room from the part of the van where bandages and antibiotics are kept in locked cabinets.

Melchiono describes the care that can be given from the inside of a mobile medical van as "Band-Aid medicine."