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Although the Roman numeral for the number 4 has always been taught to have been "IV," according to historians, the ancient Romans probably used "IIII" most of the time. This is partially backed up by the fact that early grandfather clocks displayed IIII for the number 4 instead of IV. Early clockmakers apparently thought that the IIII balanced out the VIII (used for the number 8) on the clock face and that it just looked better.
The immune system needs 9.5 hours of sleep in total darkness to recharge completely.
Most childhood vaccines are 90–99% effective in preventing disease. Side effects are rarely serious.
Fungal nail infections account for up to 30% of all skin infections. They affect 5% of the general population—mostly people over the age of 70.
The longest a person has survived after a heart transplant is 24 years.