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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.
There used to be a metric calendar, as well as metric clocks. The metric calendar, or "French Republican Calendar" divided the year into 12 months, but each month was divided into three 10-day weeks. Each day had 10 decimal hours. Each hour had 100 decimal minutes. Due to lack of popularity, the metric clocks and calendars were ended in 1795, three years after they had been first marketed.
Although puberty usually occurs in the early teenage years, the world's youngest parents were two Chinese children who had their first baby when they were 8 and 9 years of age.
In ancient Rome, many of the richer people in the population had lead-induced gout. The reason for this is unclear. Lead poisoning has also been linked to madness.
When blood is deoxygenated and flowing back to the heart through the veins, it is dark reddish-blue in color. Blood in the arteries that is oxygenated and flowing out to the body is bright red. Whereas arterial blood comes out in spurts, venous blood flows.

