Compared with bacteria, they were less complex and often more deadly. To Detrick scientists, their microscopic size offered a range of potential military advantages. A single human egg is just visible to the naked eye and has a width of about one hundred microns, or millionths of a meter. Human hairs are seventy-five to one hundred microns wide and easier to see because they are long. An ordinary human cell is about ten microns wide and by definition invisible.
Most bacteria are one or two microns wide.
They and their cousins, such as the mycobacteria, are considered the smallest of the microscopic world's fully living things. By contrast, viruses are hundreds of times smaller, and occasionally a thousand times. If bacteria were the size of cars and minivans, viruses would be the size of cell phones. One of the tinier ones, the yellow fever virus, is only two one-hundredths of a micron wide. Viruses are small because they lack most of life's usual parts and processes, such as metabolism and respiration.