Answer:
The phrase "disease is an agent of natural selection" means that those individuals which carry specific genes to resist the disease will be selected by the nature, whereas those individuals which that don't have or not capable of developing resistance genes will be excluded.
It is a well-established fact that diseases (mainly outbreaks) are best action of nature to select only those individuals which can modify there genetic material and thrive against the changing environmental conditions. Let's take the example of Coronavirus outbreak (COVID-19) this year. From the statistics we know that old people that develop the disease are most likely killed within a few days. However, young people can develop immunity against the virus easily. Hence, nature selects young people and exclude older people. This natural phenomenon is however driven by the disease.
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