Influenza A particles showing the variety of forms the virus can take. The particles are surrounded by spikes consisting of haemagglutinin and neuraminidase proteins (purple) embedded in the viral membrane. The internal helical component appears as a compressed spring-like structure where the particle has been ruptured (centre top). This internal component carries the M protein, which specifies the virus as type A or B. It is only type A, however, that undergoes the large and very rapid changes leading to new viral subtypes known as antigenic shift.
Dourmashkin/Wellcome Images
