The smallest virus as of 2014 belongs to a group of viruses known as parvoviruses. These spherical viruses are about 18 nanometers in diameter and can only be seen through an electron microscope. To put that size in perspective, one nanometer is one-billionth of a meter.
Although parvoviruses are the smallest viruses, they are not recognized as the smallest living things, because they need a host cell in order to reproduce. The smallest living thing belongs to a group of bacteria known as mycoplasmas. Mycoplasmas are about 90 nanometers in size, which is bigger than the parvoviruses but smaller than other viruses. Some of these bacteria can infect animals, plants, insects and humans.