Influenza

Influenza (flu) is caused by the influenza virus, an enveloped RNA virus. Its genome is single-stranded RNA split into eight separate segments, inside a protein capsid. This is surrounded by a lipid envelope taken from the host cell membrane.Two glycoproteins project from the envelope:
• haemagglutinin (H), which binds to receptors containing sialic acid on the surface of the host cell, so the virus can attach and enter by endocytosis
• neuraminidase (N), an enzyme that cuts new virus particles free from the host cell so they can spread.Strains are named by the types of H and N they carry, for example H1N1 or H3N2. The virus targets the ciliated epithelial cells lining the respiratory tract. Inside the cell, the viral RNA is copied and viral proteins are made using the host's ribosomes. New virions bud from the surface membrane, taking a piece of it as their envelope.The enzyme that copies the viral RNA makes frequent errors and does not proofread, so the genes for H and N mutate often. Small changes in these antigens are called antigenic drift. If two different strains infect the same cell, their RNA segments can be mixed to form a completely new strain; this is antigenic shift. Because the antigens keep changing, memory cells from past infections may not recognise new strains, so people can catch flu again and again.