Pandemic influenza is a zoonosis, as it requires introduction of avian-like gene segments in the human population.

Human influenza viruses manage to cause epidemics almost every year. The circulating viruses change their surface glycoproteins by accumulating mutations (antigenic drift) which results in variant viruses of the same subtype that are able to evade the immune pressure in the population. Every now and then, a completely new subtype of influenza A virus is introduced in the human population, which can result in an influenza pandemic. Pandemic human influenza viruses have been emerging for many centuries. Based on the genetic information of influenza viruses that have been isolated in this century, introduction of genes of the avian influenza virus reservoir obviously is required. Interspecies transmission, via another mammalian host and reassortment of avian and human influenza viruses are potential mechanisms for such an introduction. A summary of the cases in which influenza viruses containing avian-like gene segments were introduced into the human population is presented. In three cases, such infections resulted in conjunctivitis. Influenza-like illness and even pneumonia was reported in some other infections. Finally, a mortality rate of 33% was observed in the avian influenza A (H5N1) viruses that infected 18 people in Hong Kong in 1997. Although some of these viruses fulfilled some criteria of pandemic influenza viruses, they lacked the ability to rapidly spread through the human population.