Because the have hollow bones, feathers, wings, beaks, and they lay eggs
They only have one electron in their outer most shell which makes them very reactive.
While Louis Pasteur was the one that procured the germ theory of disease that explains that diseases are caused by microorganisms, the first researcher that provided direct experimental data to support the said theory is Robert Koch.
Robert Koch developed a criteria for the causality of disease from microorganisms called the Koch's postulates. The Koch's postulates indicate that:
(1) The bacteria must be present in every case of the disease
(2) <span>The bacteria must be extracted and isolated from the host with the disease and should be directly visualized and/or grown in culture
(3) There must be a healthy experimental host that will reproduce the disease once the isolated bacteria is injected
(4) The bacteria should be recoverable from the previously healthy host
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