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Vaccines cause herd immunity. If the majority of people in a community have been vaccinated against a disease, an unvaccinated person is less likely to get the disease since others are less likely to become sick from it and spread the disease.
The modern decimal position system was the invention of the Hindus (around 800 AD), involving the placing of numerals to indicate their value (units, tens, hundreds, and so on).
More than 2,500 barbiturates have been synthesized. At the height of their popularity, about 50 were marketed for human use.
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.
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.
![Pneumothorax. Figure illustrates how puncture of thoracic wall and tearing of pleural membrane allow](https://biology-forums.com/gallery/47/medium_14755_10_09_12_7_14_27_84922380.jpeg)
![Note the thickening of the intestinal wall and the erosion of the inner lining of the ileum, often s](https://biology-forums.com/gallery/47/medium_14755_30_09_12_4_42_22_9158776.jpeg)