Every year, influenza (or "the flu") does its best to confound vaccine-producing scientists. Sometimes vaccines work pretty well, and other years they miss the mark a bit. This is because flu viruses constantly change, leaving vaccine makers to take an educated guess at creating a treatment that will effectively combat that year's most common strains [source: FLU.gov].
Most of the time, seasonal flu causes significant illness and discomfort in the form of fever, chills and body ache, but this highly contagious illness usually isn't life-threatening. Certain groups are at higher risk, like people over age 65, young children, people with compromised immune systems and pregnant women [source: WHO].
Flu outbreaks have occurred that devastated populations, however. The worst on record was probably the 1918 outbreak that killed approximately 40 million people worldwide [source: National Institute of Allergy and Infectious Diseases]. Science today appears to be on our side, thankfully. In 2014, there were 250,000 to 500,000 deaths from influenza worldwide [source: WHO].