Potential donors are evaluated to ensure their blood
is safe to use. Donated blood undergoes screening
includes testing for diseases that can be transmitted by a
blood transfusion, including HIV and viral hepatitis.
The donor is also asked about medical history and
given a short physical examination to make sure that
the donation is not hazardous to his or her health. The
frequency of blood donation can vary from days to
months and also depend on the laws of the land.