Bacteria are organisms that can survive by themselves.
They are large and usually have a cell wall that can be stained for identification. In the later decades of the 19th century, it became obvious that some infective agents were smaller than bacteria and could not be seen with a microscope. Thus, while the 'poisonous' nature of the organisms causing small pox and rabies was well recognized, the organisms themselves could not be found. These types of germs were subsequently classified as viruses. Viruses are the smallest of the infectious organisms and cannot grow by themselves but have to live in the cells of an infected person.