Human flu is a term used to refer to influenza cases caused by Orthomyxoviridae that are endemic to human populations (as opposed to infection relying upon zoonosis). It is an arbitrary categorization scheme, and is not associated with phylogenetics-based taxonomy. Human flu-causing viruses can belong to any of three major influenza-causing Orthomyxoviruses — Influenza A virus, Influenza B virus and Influenza C virus.

The annually updated trivalent influenza vaccine contains two hemagglutinin (HA) surface glycoprotein components from Influenza A virus strains and one from B influenza.

Most human flu is a non-pandemic flu that is slightly different from the main human flus that existed in last year's flu season period. This type of flu is also called \"common flu\" or \"seasonal flu\" or \"annual flu\". It causes yearly flu epidemics that are generally not deadly except to the very old or very young.