The term myxovirus was coined for a group of enveloped RNA viruses that have the ability to adsorb onto mucopro-tein receptors on erythrocytes, causing hemagglutination. It included influenza, mumps, parainfluenza, and Newcastle disease viruses.
All these viruses were grouped under myxovirus (myxa meaning mucus) due to their affinity to mucins. The influenza, mumps, parainfluenza, and Newcastle disease viruses were initially grouped in this class. However, two separate families were created subsequent to the recognition of clear differences between these viruses. These two fami-lies are: Orthomyxoviridae, consisting of influenza viruses, and Paramyxoviridae, consisting of parainfluenza, mumps, measles, respiratory syncytial, and Newcastle disease viruses. Differences between orthomyxoviruses and paramyxoviruses are summarized in Table 61-1.
The genus Orthomyxovirus includes influenza viruses, the causative agents of worldwide epidemics of influenza. Human diseases associated with influenza virus are presented in Table 61-2.
Influenza viruses are classic respiratory viruses. They cause influenza, an acute respiratory disease, with well-defined systemic symptoms. Influenza is an acute infectious disease of the respiratory tract that occurs in sporadic, epidemic, and pandemic forms.