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. The RNA or DNA contains the information that eventually tells the cell to make more viruses. The protein coat protects the RNA or DNA from being destroyed and helps the virus to attach to the cells it infects. A good place to look at some nice simple cartoons of viruses are found in Baron's online Medical Microbiology textbook.