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Hippocrates noted that blood separates into four differently colored liquids when removed from the body and examined: a pure red liquid mixed with white liquid material with a yellow-colored froth at the top and a black substance that settles underneath; he named these the four humors (for blood, phlegm, yellow bile, and black bile).
Vaccines prevent between 2.5 and 4 million deaths every year.
After a vasectomy, it takes about 12 ejaculations to clear out sperm that were already beyond the blocked area.
Autoimmune diseases occur when the immune system destroys its own healthy tissues. When this occurs, white blood cells cannot distinguish between pathogens and normal cells.
If all the neurons in the human body were lined up, they would stretch more than 600 miles.