Plant cells and animal cells are much larger than bacterial cells. On average, animals cells range from 10 to 30 micrometers in length, while plant cells average between 10 and 100 micrometers in length. By comparison, bacterial cells are less than 2 micrometers long.
A micrometer, also called a micron, is a unit of measure equalling a millionth of a meter. Objects as small as 10 to 30 micrometers are invisible to the naked eye and can only be viewed with a microscope. (For the purposes of comparison, a typical period at the end of a sentence measures approximately 1,000 micrometers across.) Bacterial cells are much smaller than plant and animal cells, because bacterial cells are much simpler. They lack nuclei and other cell structures that take up room in the more complex cells of plants and animals. Viral cells are even smaller than bacterial cells. A rhinovirus cell is only 30 nanometers long. Trillions of bacterial cells inhabit the normal, healthy human body. In fact, there are approximately 10 times more bacterial cells in the human body than there are human cells, yet because of their small size, all of those bacterial cells only fill a single half-gallon jug.