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Human neurons are so small that they require a microscope in order to be seen. However, some neurons can be up to 3 feet long, such as those that extend from the spinal cord to the toes.
In most cases, kidneys can recover from almost complete loss of function, such as in acute kidney (renal) failure.
The immune system needs 9.5 hours of sleep in total darkness to recharge completely.
There can actually be a 25-hour time difference between certain locations in the world. The International Date Line passes between the islands of Samoa and American Samoa. It is not a straight line, but "zig-zags" around various island chains. Therefore, Samoa and nearby islands have one date, while American Samoa and nearby islands are one day behind. Daylight saving time is used in some islands, but not in others—further shifting the hours out of sync with natural time.
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.