What is the approximate diameter of a typical influenza virus?

The approximate diameter of a typical influenza virus is 100 nanometers. That is about one ten-thousandth of a millimeter, far too small to see with a standard light microscope.

Influenza virions are enveloped particles, meaning their outer membrane is borrowed from the host cell during viral release. Embedded in that membrane are the prominent proteins hemagglutinin and neuraminidase, commonly known as HA and NA. These surface proteins help the virus attach to cells and release newly formed particles.

The 100-nanometer figure is an approximation rather than a fixed measurement. Influenza particles commonly fall in the range of roughly 80–120 nanometers across. Some particles are filamentous instead of roughly spherical or ellipsoidal; those can be much longer, so “diameter” does not describe their entire length.

Influenza A viruses also have an eight-segment negative-sense RNA genome. The segmented genome helps explain how different influenza viruses can exchange gene segments when they infect the same host cell, a process that can produce substantially different combinations of viral traits.