On a bracingly cold day in March, four people set out from Ukraine’s capital, Kyiv, to deliver lifesaving medicines, heating devices and food to the besieged residents of Chernihiv, in the northeast.

Only one survived.

During a quick stop en route, the convoy was hit by Russian shelling. Two of the four people died instantly. A third was pelted by fragments and died a half-hour later. The survivor was a man who had stepped away from the vans to relieve himself.

Among the dead: 21-year-old Aanastasiia Tagirova, who had wanted to go to Chernihiv to reunite with her boyfriend, and her cat.

The ill-fated trip, on March 30, was organized by 100% Life, a large nonprofit group that serves Ukrainians living with H.I.V. It was not the group’s first mission to Chernihiv, nor its last.

Undeterred, the organization’s staff and volunteers have continued to make forays into Chernihiv, learning to be nimble and unobtrusive to Russian eyes. So far, they have delivered enough medicines to treat the 1,800 people in Chernihiv known to be living with H.I.V., although an unknown number may have fled the city or been killed.

Public health groups like these are going to extraordinary lengths to help their compatriots and to preserve Ukraine’s hard-won progress against H.I.V., tuberculosis and other scourges. While they have always committed to saving lives, the goal has come to mean something entirely different since the invasion.

“Struggling and fighting for life is our principle,” Dmytro Sherembei, who heads 100% Life, said through a translator in a recent interview. “We always have to be ready to fight for life under any circumstances and any conditions.”

Experts have warned that wars nearly always lead to public health crises. Pathogens find easy targets among large groups huddled closely together in basements and refugee camps, children who miss routine vaccinations and patients who lose access to medications.

Interruptions in treatment for H.I.V. and TB can breed versions of the pathogens that are resistant to the medications. Ukraine and its neighbors already represent the global epicenter of drug-resistant tuberculosis.

On March 25, volunteers from 100% Life loaded two vans with a half ton of medicines — including liquid formulations of H.I.V. treatments for children — clothes and food. The only bridge to Chernihiv had been destroyed by bombing, so they drove the vans to the banks of the Dnipro River, transferred the cargo to a boat and unloaded it on the other side. They returned with 34 people fleeing Chernihiv.

On their second trip, on March 30, volunteers again loaded two minivans with food and medicines, and this time heating devices for freezing residents. They were joined by three vans from an evangelical church hoping to evacuate some of their members from Chernihiv.

The shelling incinerated four of the vans and partially destroyed the fifth. Mr. Sherembei said three people from the church group had been brought to a hospital, but he did not know their fates.

“The Russians were bombing us, they were shelling us, knowing full well that it was a humanitarian convoy,” he said.

The organization lost two volunteers: Bohdan Stefanyshyn, 40, and Oleksii Antonov, 28. Yurii Luniov, the 41-year-old volunteer who had stepped away into the bushes, was spared.

Mr. Sherembei seemed particularly distraught as he talked about Ms. Tagirova. No entreaties from him or from her friends and family had dissuaded her from joining the trip, he said.

Even after the tragedy, abandoning the missions was not an option — not when there were people, including many children, who needed the medicines, he said. So the volunteers thought of ways to make themselves less conspicuous, and have made three trips so far.

Figuring that one van would draw less attention than a convoy, and that the Russians might be unlikely to use precious missiles on a single target, they swapped the two small vans for a large dull-colored one. They no longer travel with the church group. They packed solar panels, candles for people living out of dank basements and food. The aid workers could bring back only about 10 people.

By the time of the March 30 trip, the Ukrainian military had fashioned a makeshift pontoon bridge across the river that the van could traverse. Not having to unload and load the cargo slashed the time it took to cross the river from four hours to just 10 minutes.

On the Chernihiv side, the five 100% Life workers who opted to stay behind in Chernihiv, as well as staff from an infectious disease unit in one of the few hospitals that’s still standing, stood ready to collect and distribute the supplies.

Communicating with these helpers held its own challenges. Many of them were holed up in basements except for brief stints, and their cellphones were operational for just one unpredictable hour each day.

Once the plan was finalized, the news had to be circulated among patients by word of mouth. (In the past few days, re-established power stations have eased these conditions somewhat, Mr. Sherembei said.)

While Chernihiv residents with H.I.V. have enough treatments for now, the medicines are in short supply. The United States President’s Emergency Plan for AIDS Relief has scoured the world to secure spare stocks of H.I.V. treatments for Ukraine. Those drugs are being delivered through Poland and Romania, according to Unaids.

But moving the medications through war-torn parts of the country has been more complicated. “Only now are we starting to get those deliveries that were sent out a month ago to make up for that shortage,” Mr. Sherembei said.

Many of 100% Life’s staff consider themselves soldiers. The day after our interview, Mr. Sherembei left to fight on the front lines. He said he did not feel he could make any other choice.

“The enemy is trying to create panic within the population,” he said. “Of course, there’s always a risk that people will die, that our people will die when delivering the goods. But we have to help somehow.”