virus including fever (38–40 °C), dry cough, rhinorrhea (nasal discharge), headache, muscle pain, and achiness. The virus may lead to more severe infections such as bronchitis and pneumonia. After an individual becomes infected, the immune system develops antibodies against that infectious agent. This is the body's main source of protection. Most children between five and ten years old have already produced antibodies for influenza virus C. As with all influenza viruses, type C affects individuals of all ages, but is most severe in young children, the elderly and individuals with underlying health problems. Young children have less prior exposure and have not developed the antibodies and the elderly have less effective immune systems. Influenza virus infections have one of the highest preventable mortalities in many countries of the world. Structure and variation Influenza viruses, like all viruses in the family Orthomyxoviridae, are enveloped RNA viruses with single stranded genomes. The antigens, matrix protein (M1) and nucleoprotein (NP), are used to determine if an influenza virus is type A, B, C, or D. The M1 protein is required for virus assembly and NP functions in transcription and replication. These viruses also contain proteins on the surface of the cell membrane called glycoproteins. Type A and B have two glycoproteins: hemagglutinin (HA) and neuraminidase (NA). Types C and D have only one glycoprotein: hemagglutinin-esterase fusion (HEF). These glycoproteins allow for attachment and fusion of viral and cellular membranes. Fusion of these membranes allows the viral proteins and genome to be released into the host cell, which then causes the infection. Types C and D are the only influenza viruses to express the enzyme esterase. This enzyme is similar to the enzyme neuraminidase produced by Types A and B in that they both function in destroying the host cell receptors. Glycoproteins may undergo mutations (antigenic drift) or reassortment in which a new HA or NA is produced (antigenic shift). Influenza virus C is only capable of antigenic drift whereas Type A undergoes antigenic shift, as well. When either of these processes occur, the antibodies formed by the immune system no longer protect against these altered glycoproteins. Because of this, viruses continually cause infections. Identification Influenza virus C is different from Types A and B in its growth requirements. Because of this, it is not isolated and identified as frequently. Diagnosis is by virus isolation, serology, and other tests. Hemagglutination inhibition (HI) is one