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For about 100 years, scientists thought that peptic ulcers were caused by stress, spicy food, and alcohol. Later, researchers added stomach acid to the list of causes and began treating ulcers with antacids. Now it is known that peptic ulcers are predominantly caused by Helicobacter pylori, a spiral-shaped bacterium that normally exist in the stomach.

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About 3% of all pregnant women will give birth to twins, which is an increase in rate of nearly 60% since the early 1980s.

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Cancer has been around as long as humankind, but only in the second half of the twentieth century did the number of cancer cases explode.

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During the twentieth century, a variant of the metric system was used in Russia and France in which the base unit of mass was the tonne. Instead of kilograms, this system used millitonnes (mt).

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Long-term mental and physical effects from substance abuse include: paranoia, psychosis, immune deficiencies, and organ damage.

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Change in the frequency of a lethal recessive allele, a Change in the frequency of a lethal recessive allele, a
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