The answer might be gene pool?
Answer: germ theory
Explanation: Germ theory of disease is a theory in biology. It says that small organisms (called germs), also known as microbes, cause diseases. Most, but not all diseases are infectious diseases. The germ theory states that small organisms cause a reaction in the body of those who are infected. The body's reaction to infection is called a disease.
Reference: Wikipedia
Number of people per square mile.
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Answer:
Viruses are too small and simple to collect or use their own energy – they just steal it from the cells they infect. Viruses only need energy when they make copies of themselves, and they don't need any energy at all when they are outside of a cell.
Explanation:
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