Plagues and other great diseases are important parts of history and some settings. They defy man and may even defy God in their swift and terrible punishment upon the body of society.
Silk Armor? The Mongols wore silk undergarments under layers of leather armor. Why? Because silk is very strong. If an arrow hit them and made it through their armor, it would usually not have enough energy to puncture the silk. It could still enter their body but, because the silk would not break, the arrow's barbs could not do their work and the arrow could easily be removed leaving a relatively clean wound considering normal arrow wounds.