The flu kills an average of 20,000 Americans annually. The flu is easy to prevent, but tough to treat. Since influenza is viral, antibiotics are ineffective. Preventing influenza is better than treating it. The flu virus keeps changing.
Influenza viruses change constantly.
Once the viruses change, antibodies in the immune system produced by prior bouts of the disease don't recognize and attack them. Illness may result. The Centers for Disease Control in Atlanta keeps track of influenza viruses to reduce the chance of another unexpected mutation. Each year's influenza vaccine is formulated to include the strains currently circulating. Mutated influenza viruses can cause serious pandemics.