Viruses are very small forms of life. In fact people still argue over whether viruses are really alive. A nanometer is 0.000001 of a millimeter. So in other words you can place 25,000,000 nanometers in an inch. If the biggest virus is 300 nm then you could fit 83,333 of that virus in an inch.
Viruses are major freeloaders.
They can not make anything on their own. To reproduce they must infect other living cells. Viruses infect bacteria, parasites, fungi, plants, animals, and humans. No one escapes them. The RNA or DNA helps the virus take over the the cell it infects.