Because their soft tissue and bones decay or get eaten before a fossil can be made.
Individual and public health are affected by environmental and genetic factors is a statement which reflects reality. Firstly, susceptibility to certain diseases usually or often runs in a family so that if there is say, diabetes in the father and grandmother for example, the son or daughter has more chances of getting it so this is a genetic susceptibility.. Also, some families are more prone to cancer so that given the right circumstances, cancer can develop. So it is usually a combination of both genetic and environmental causes that results in a disease. An example of public health could be when smallpox was deliberately introduced to the First Nations people in British Columbia, Canada back in the 1800's to decimate them and remove their resistance to the gold mining hordes. Since the First Nations had no anti-bodies to counteract such a disease unlike the white men who introduced it to them, then they easily fell prey to it and died within days of contracting it.
There is approx 4.7-5.5 liters of blood in his or her vascular system.
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