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Blood is approximately twice as thick as water because of the cells and other components found in it.
An identified risk factor for osteoporosis is the intake of excessive amounts of vitamin A. Dietary intake of approximately double the recommended daily amount of vitamin A, by women, has been shown to reduce bone mineral density and increase the chances for hip fractures compared with women who consumed the recommended daily amount (or less) of vitamin A.
Pubic lice (crabs) are usually spread through sexual contact. You cannot catch them by using a public toilet.
Intradermal injections are somewhat difficult to correctly administer because the skin layers are so thin that it is easy to accidentally punch through to the deeper subcutaneous layer.
Bacteria have flourished on the earth for over three billion years. They were the first life forms on the planet.