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Most childhood vaccines are 90–99% effective in preventing disease. Side effects are rarely serious.
Sperm cells are so tiny that 400 to 500 million (400,000,000–500,000,000) of them fit onto 1 tsp.
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.
Giardia is one of the most common intestinal parasites worldwide, and infects up to 20% of the world population, mostly in poorer countries with inadequate sanitation. Infections are most common in children, though chronic Giardia is more common in adults.
According to the National Institute of Environmental Health Sciences, lung disease is the third leading killer in the United States, responsible for one in seven deaths. It is the leading cause of death among infants under the age of one year.