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If they are dying that early I'd guess that you haven't yet got as far as L2 medicine. First, make that a priority. Also, keep an eye on your villagers' health bars and make sure that anyone sick is cured as soon as possible to avoid disease damage. There is a special stew that will restore even badly-weakened villagers to full health.

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There are lots of tips in the Guides forum that will help you develop your villagers' skills, and I also suggest reading through the posts in this forum, which also contain useful tips.