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There are more nerve cells in one human brain than there are stars in the Milky Way.
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.
The immune system needs 9.5 hours of sleep in total darkness to recharge completely.
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.
HIV testing reach is still limited. An estimated 40% of people with HIV (more than 14 million) remain undiagnosed and do not know their infection status.