influenza viruses, spikes mutate frequently. Types A, B, & C. Virus destroys the epithelial lining of the respiratory tract thus allowing secondary bacterial infection to enter. Type A strains are responsible for pandemics that occur at intervals of 10 to 20 years. The B strains cause local outbreaks and epidemics less often. Type C strains are rare. Pneumonia is a common secondary infection. A yearly vaccine, based on the predominant A and B strains, is about 70% effective.