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There are over 65,000 known species of protozoa. About 10,000 species are parasitic.
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).
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.
Aspirin is the most widely used drug in the world. It has even been recognized as such by the Guinness Book of World Records.
The average adult has about 21 square feet of skin.