The first thing I hear about when I arrive at the National Mall is the naked guy. I am standing with a group of 20-some activists with red crosses taped to their shoulders and backs, in front of a large yurt that’s been designated the medical clinic for the protest. Around us are crammed countless thousands of protesters, stretching shoulder-to-shoulder all the way from the Capitol building to the Washington Monument. In their hands are multicolored placards attesting to their diversity: wisconsin for justice, the raging grannies, another lesbian mom for peace.

And then there’s the naked guy. Apparently one of the activists at today’s antiwar protest has stripped down to his Speedo in the freezing cold and is encouraging others to write radical slogans on his bare flesh. I get this news from Earthworm, a young woman with strawberry hair, glasses, and a bulky medic vest, who will be one of my partners today. I mentally run through the risks he’s running — hypothermia, frostbite (can you get that down there?) — before asking if there’s anything we can do. "Someone has gone out to look for him," says Earthworm, "but we should all keep our eyes out."

This wasn’t something I’d expected when I signed up to serve as "street medic" for the antiwar protest in Washington last month. Then again, during the three days of training in Boston, I had been told specifically to expect the unexpected.

Street medics, also called "action medics," are a ragtag collection of doctors, EMTs, and ordinary civilians who have taken it upon themselves to offer medical care for crowds at progressive rallies and street actions. Ever since the tear gas started flying and the riot cops began cracking skulls in Seattle three years ago, these anarchist Florence Nightingales have been an increasing presence at globalization and antiwar protests. Because professional ambulance paramedics are frequently not allowed beyond police lines, the street medics are often the only health-care providers on the scene.

This is my first time running as a medic, but not my first experience with activist medicine. That occurred in April 2001, during a protest of the Free Trade Area of the Americas agreement in Quebec City, when police turned the city into a walled fortress, complete with a six-foot-high chainlink fence and hundreds of riot cops with submachine guns. I was standing in the middle of a plaza with a friend when a canister of pepper spray landed at my feet. Suddenly, my nose and throat were on fire; my eyes clenched tight as I stumbled blindly away. Out of nowhere, a stranger grabbed me and led me behind a wall into fresh air, pried open my eyes and flushed them with cold water, then stayed nearby until I could breathe again. Thankful doesn’t even begin to describe my gratitude to whomever helped me that day.

So when the opportunity presented itself, I found myself joining a dozen other activists in a Northeastern University classroom last month for training hosted by the Boston Area Liberation Medics — a/k/a BALM Squad — one of the most active of the various medical collectives scattered throughout the country. There we absorbed the basics of first aid in two nine-hour sessions, practiced eye flushing on ourselves, and learned the rules that collectively govern this loose confederation of activists.

"People who haven’t had the training, the whole training, won’t run as a medic," says Eowyn Reiche, a Boston family physician and BALM Squad member who, along with Earthworm, will be my partner today in DC. Medics are organized in teams of at least two, with the less experienced paired with those who’ve seen more time in the street. Earthworm — whose real name is Dell MacLean — is a licensed EMT and a member of the local collective DC Action Medical Network (DAMN). Together we are "Team Sunshine," an optimistic name on a day like today, when the high is expected to be 28 degrees.

In fact, this cold is the kind that turns your eyeballs to ice cubes and your toes to lead, that gets inside your bones no matter how many layers of clothing you have on. Personally, I’m wearing four. But some members of the crowd are clearly unprepared for this unseasonably frigid DC weather. Official estimates of attendees range from 100,000 to 500,000, making this the largest antiwar protest since the Vietnam era. With only a couple dozen medics for the entire crowd, we’ll have our hands full.

For the next two hours, we crisscross the area, keeping our eyes peeled for anyone suffering from the cold. Our main worry is hypothermia, or abnormally low body temperature, which can cause dizziness and disorientation and, counter-intuitively, a feeling of being too warm. In an effort to alert as many people as possible to the dangers and symptoms, we snake our way through the crowd to the main stage, where we ask to make an announcement about what people can do to prevent getting hypothermia themselves (drink water, keep heads covered).

As we’re waiting to speak, a woman shyly asks us if we have a Band-Aid. She’s cut her finger; I hold a piece of gauze over her bloody knuckle and Earthworm tapes it into place. It’s not exactly an ER moment, but already I’m feeling the satisfaction that comes with being in the right place at the right time. "More often than not, there isn’t a whole lot that we end up doing," says Reiche. "We’re here for moral support and taking care of the crowd." Still, it’s an obvious high point for her when she can get up on stage in front of hundreds of thousands of people and tell them to drink fluids and dress in layers.

By the time we get to the warming bus that protest organizers have set up, it’s already packed with people. We hand out hats and scarves that were donated by a man from New Jersey, and give candied ginger (a circulation stimulant) to the coldest on board. Earthworm gives some socks to a girl from Connecticut who has a hole in her shoe. I’m surprised by how much our red crosses give us an air of authority, inspiring people to ask us for information about everything from the time of the march to the location of the bathrooms. "Sometimes I feel like Ms. Fix-It," says Reiche. "People look to us for answers on almost everything." She’s only been running as a medic for nine months, though she’s been going to demonstrations for longer. "It seemed like the perfect way to combine my formal training and political interests," she explains. "I had taken all these years of training, and it seemed like I could use that to help radical and progressive movements." When Reiche first started as a medic, she was struck by the lack of hierarchy in the movement, and the emphasis on listening to what patients want instead of forcing treatments on them. "It was like finally finding a way to deliver health care in a way that I really believed in."

Though some of them lack formal credentials, many action medics actually see their movement as more rigorous than traditional emergency medicine. "We take it more seriously," asserts Brian Dominick, an EMT and street medic from New York. "They are just teaching you to pass an exam. We go in knowing that we’ll be working with these people in the streets." That doesn’t mean street medics don’t value professional paramedics, however. Over and over the training stressed that our primary job is to get victims to further help, usually by calling 911. "It’s not like we are keeping the EMTs at bay and saying, ‘We found them first,’ " says Dr. Michael Greger, the founder of BALM Squad. Oftentimes, it’s just that EMTs aren’t available, since police won’t let them beyond their cordon into what they term an "unsecured area."

"There’s that space between where things happen and where other care is [available] that we fill very nicely," says Adrianne "Ace" Allen, a medic who led my training in Boston and who’s been hitchhiking around the country for the past nine months, setting up clinics and trainings.

When in doubt, all medics stress their number-one motto, "Do no harm," which essentially means don’t treat any medical problem unless you’re sure you won’t make things worse. "We are extremely wary of doing things that might have a negative effect," says Reiche. Not that there’s much risk of doing harm today. As the march gets going down the middle of the street, we nervously scan the lines of riot cops, noting which officers are holding nightsticks — notorious for causing head injuries — and which cops have pepper-spray canisters. But they stay in formation, and even the black-bloc anarchists refrain from antagonizing them. The most serious injury we come across is a skinned knee, which we wash out and bandage, sealing the ripped pant leg against the cold with duct tape.

"The best times are the times like these," says Earthworm, "when you can be the crowd’s caretaker, when you can give out socks or ginger and people are like, ‘Oh God, thank you.’ " Her own first experience as a medic was very different. After completing a training in North Carolina in 2001, she arrived in Quebec City not even intending to run as a medic. Then the tear gas started flying and she realized she could use her skills to benefit her friends and other activists. "Ever since then, I’ve been passionate about being a medic," Earthworm says. "Maybe it was the gratitude or the ability to take back a situation, but I fell in love with doing that stuff." When she returned from the protest she took a Red Cross first-aid course, then a basic EMS course. Recently she applied to be a volunteer EMT. "The street-medic training totally changed my life," she says. "I have a passion now. I’m so intensely grateful to whoever started this."