Which of the following microorganism commonly shows antigenic drift in its pathogenicity?

Influenza viruses are remarkable because of the frequent antigenic changes that occur in hemagglutinin (HA) and neuraminidase (NA). Antigenic variants of influenza virus have a selective advantage over the parental virus in the presence of antibody directed against the original strain. This phenomenon is responsible for the unique epidemiologic features of influenza. Other respiratory tract agents do not display significant antigenic variation. Also know: Minor antigenic changes are termed antigenic drift; major antigenic changes in HA or NA, called antigenic shift, result in the appearance of a new subtype. Antigenic shift is most likely to result in an epidemic. Antigenic drift is caused by the accumulation of point mutations in the gene, resulting in amino acid changes in the protein. Ref: Brooks G.F., Carroll K.C., Butel J.S., Morse S.A., Mietzner T.A. (2013). Chapter 39. Ohomyxoviruses (Influenza Viruses). In G.F. Brooks, K.C. Carroll, J.S. Butel, S.A. Morse, T.A. Mietzner (Eds), Jawetz, Melnick, & Adelberg's Medical Microbiology, 26e.