Maai Mahiu, Kenya — The trucks arrive nightly, massive and wheezing, their cargo bays loaded down with food, clothes, electronics, and petrol collected from the port of Mombasa and bound for the continent’s landlocked interior. Each evening, hundreds of them pull off the road at this trading post 53 kilometers northwest of Nairobi, their drivers lured by a growing collection of hotels, bars, and restaurants splayed out along the busy highway.

And by something else too.

“I make my living from these truck drivers,” says Elizabeth Wambui, a commercial sex worker who, like many in the town’s hospitality industry, moved here with the promise of growing business opportunities in the transit boomtown. And so far, she hasn’t been disappointed.

But now another group has recognised the strategic value of Maai Mahiu’s location as well — a health charity called North Star Alliance, which since 2010 has operated a nocturnal HIV “wellness center” in a repurposed blue shipping container deposited in the truck park here.

The small clinic is one of eight built since 2009 along the spine of Kenya’s “northern corridor,” the main trade route between the country and its landlocked neighbours and a hub for HIV transmission in the region.

The goal of the mobile clinics is simple, says says Snowrinna Ndida, an outreach coordinator with the North Star Alliance Wellness Center in Maai Mahiu. It is to reach two groups of people particularly vulnerable to infection — truck drivers and commercial sex workers — at a place where both live and work.

To do that, the clinics also try to keep the same hours as their clients, which has another benefit too. They can come for free HIV counseling, testing, and treatment literally under the cover of darkness.

“We prefer visiting these clinics at night so that we minimize chances of being seen by people who are well known to us,” says Herman Remi, 35, a long distance truck driver who says he contracted HIV in 2009 while on the road in Uganda. Trucking, he explains, keeps men away from home for such long stretches of time — in his case a month or more at a go — that having partners besides their spouses is almost inevitable.

Indeed, for decades, both commercial sex workers and long-haul truck drivers have been cast by many as villains of the HIV epidemic, maligned for their role in rapidly spreading the virus through their multiple partners and sprawled geographies. But now groups like North Star are flipping that script.

If these groups — often referred to in public health speak as “key populations” — are at high risk for catching and spreading HIV, they reason, that also means they are likely to be at the core of any successful effort to keep it contained.

That’s why although North Star offers HIV treatment, it focuses most of its efforts on prevention, says Stephen Gichina, a clinician and site coordinator at the North Star clinic here.

“We normally have a large batch of condoms at our disposal,” he says. “At the moment we have in our stores over 250,000 condoms, which we expect to be out of stock in less than two month’s time.”

The clinic has a roving band of eight peer educators recruited from the local sex worker community. Among them is Wambui, the veteran sex worker, who is also herself HIV-positive.

“When I realised [ten years ago], I thought it was the end of me, but after undergoing several counseling sessions I realised I can live longer,” she says, clutching on a packet of condoms at the North Star Alliance clinic.

Now, she tries to offer the same assurances to her colleagues, making nightly visits to some 63 sex workers scattered across 24 local cruising spots, where she distributes free condoms and lubricant, counsels and answers questions about safer sex, and refers new sex workers to the clinic for check-ups. To date, North Star says it has been able to reach 524 sex workers in Maai Mahiu with the help of these peer educators.

And their efforts appear to be paying off.

According to the 2016 Kenya Aids Progress Report, Nakuru County — where Maai Mahiu is located — saw its rate of new HIV infections plunge from 4127 in 2013 to 801 in 2015, one of the largest percentage drops in the entire country over that period.

“Sex workers are now more aware of how HIV is transmitted, and are quick to even report cases of condom bursting,” says Ndida, the North Star outreach coordinator.

Still, the county continues to rank in the top third for HIV prevalence nationwide, and stigma around the disease remains high, according to the “stigma index” of the progress report.

And while the North Star clinics have been particularly successful in reaching out to sex workers, they still need to make more significant efforts to include truck drivers in their outreach as well, says Nicholas Mbugua, the secretary general for the Kenya Long Distance Truck Drivers and Allied Workers union. After all, more than half of people who visit the clinic for testing and treatment don’t live in Nakuru county, according to North Star. And most of those just passing through are likely drivers.

“The clinic’s closing time is unpredictable — you will [sometimes] find them closing around 8 pm while our most of our drivers [arrive] past that time,” says Mbugua.

North Star has recently begun to target drivers through a programme it dubs “Star Driver,” which provides free medical checkups, defensive driving lessons, renewal of driver’s licenses, reflector jackets and monetary rewards to drivers who make regular visits to its clinics.

“We have seen a significant number of drivers visit our center since the programme was initiated two years ago,” says Gichina.

The clinic as a whole, meanwhile, continues to do a brisk business — nearly as busy most nights as the clogged Nairobi-Naivasha highway that borders it.

A truck driver on his way to Uganda, explains. He’s come to collect condoms from the clinic. “You know, we’re only human. It’s easy to fall into temptations if you’re away from home for so long.”

Early the next morning, while the suns starts to rise over the Great Rift Valley, where Maai Mahiu is situated, the 35- year old driver gets ready for the next part of his long-distance journey. He gets into his truck, and says: “These condoms will help me to protect myself against HIV.”