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The first war in which wide-scale use of anesthetics occurred was the Civil War, and 80% of all wounds were in the extremities.
Not getting enough sleep can greatly weaken the immune system. Lack of sleep makes you more likely to catch a cold, or more difficult to fight off an infection.
Vaccines cause herd immunity. If the majority of people in a community have been vaccinated against a disease, an unvaccinated person is less likely to get the disease since others are less likely to become sick from it and spread the disease.
Your skin wrinkles if you stay in the bathtub a long time because the outermost layer of skin (which consists of dead keratin) swells when it absorbs water. It is tightly attached to the skin below it, so it compensates for the increased area by wrinkling. This happens to the hands and feet because they have the thickest layer of dead keratin cells.
There are 60,000 miles of blood vessels in every adult human.