ANTIGENIC

SMALL STEPS: As the influenza virus multiplies and spreads from host to host, it undergoes small genetic changes, called “antigenic drift.” So many changes may eventually accumulate that the virus essentially becomes a new entity, requiring the immune system to make new antibodies. GIANT LEAPS: Influenza viruses can also change

from different strains recombine into a new form—an event called “antigenic shift.” These

likely to produce virulent new strains that can create outbreaks in people or animals who have no natural immunity.