All living creatures, including humans, are made of cells. The majority of life forms exist as single cells that perform all functions to continue independent life. Some cell structures, cell organelles and particularly bacteria and viruses are commonly too small to be fully observed with an optical microscope. Therefore, an electron microscope is required. Since samples examined with an electron microscope are exposed to very high vacuum, it is impossible to view living cells. The sample preparation for electron microscopy requires that living cells be killed, frozen, dehydrated, and impregnated with heavy metals.