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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.
Only 12 hours after an egg cell is fertilized by a sperm cell, the egg cell starts to divide. As it continues to divide, it moves along the fallopian tube toward the uterus at about 1 inch per day.
Most childhood vaccines are 90–99% effective in preventing disease. Side effects are rarely serious.
In the United States, there is a birth every 8 seconds, according to the U.S. Census Bureau's Population Clock.
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.