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Pope Sylvester II tried to introduce Arabic numbers into Europe between the years 999 and 1003, but their use did not catch on for a few more centuries, and Roman numerals continued to be the primary number system.
There are over 65,000 known species of protozoa. About 10,000 species are parasitic.
Most childhood vaccines are 90–99% effective in preventing disease. Side effects are rarely serious.
More than 2,500 barbiturates have been synthesized. At the height of their popularity, about 50 were marketed for human use.
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.