Over the past decade, Mexico has expanded access to medical services as successive governments have raised spending toward reaching the far-off goal of universal health coverage. But the sort of labor-intensive care that Mr. Castro provides is beyond the scope of rudimentary health centers in the countryside or the Hospital de las Culturas, a community hospital that opened here three years ago.

Don Sergio, as people here call him, spends much of his time patiently cleaning and bandaging wounds caused by burns or diabetes. He accepts no money from his patients, because “then they can be calm and they are more motivated to heal quickly,” he said.

“The ability, the gift that God gave me to do this — that is what gives me results,” he said.

When he can gather together enough from the donations that support him and his work — including from American expatriates living in Mexico — he helps villages build schools and treat their water.

Now his concern is the López family. Although Mr. López, 29, is covered by government insurance, there is no ambulance to take him for rehabilitation an hour away, and his disability claim is ensnared in the bureaucracy. Mr. Castro has been using donations to pay for physical therapy, but money is running out. He has been thinking ahead: planning for a tiny convenience store that Mr. López could run from home.

“This is a special case that I am committed to,” Mr. Castro said of his patient. “It’s indispensable for him to change psychologically, not to feel useless, to have some resources.”