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Related to Myxoviruses: orthomyxoviruses
a group of viruses that cause diseases in mammals and birds. The virions are spherical and consist of a nucleoid (a ribonucleoprotein with a spiral arrangement of proteins) and an external membrane containing proteins, lipids, and carbohydrates. Orthomyxoviruses (the ribonucleic acid has a molecular weight of approximately 3 million and consists of several fragments) include the causative agents of human influenza and fowl plague; paramyxoviruses (the ribonucleic acid is a molecule with a molecular weight of 6–7.5 million) include the causative agents of Newcastle disease and measles.