CPEs are important aspects of a viral infection in diagnostics. Many CPEs can be seen in unfixed, unstained cells under the low power of an optical microscope, with the condenser down and the iris diaphragm partly closed. However, with some CPEs, namely inclusion bodies, the cells must be fixed and stained then viewed under light microscopy. Some viruses' CPEs are characteristic and therefore can be an important tool for virologists in diagnosing an infected animal or human. The rate of CPE appearance is also an important characteristic that virologists may use to identify virus type. If CPE appears after 4 to 5 days in vitro at low multiplicity of infection, then the virus is considered slow. If the CPE appears after 1 to 2 days in vitro at low multiplicity of infection, then the virus is thought to be rapid. Inoculations always occur at low multiplicity of infection because at high multiplicity of infection, all CPEs occur rapidly.