If your villagers show a status of "Sick" when you look at the details screen, you can heal them by dragging another villager to them. Even a sick villager can heal another villager. If it doesn't work the first time, try again. Sometimes a villager will "resist" healing attempts by a specific doctor. If that happens, try a different villager. Villagers who show a status of "weakened" will generally improve in health over time unless they are sick, elderly or continue to suffer on-the-job injuries.
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Barbara
Unicorn
Last Day of Work