Each year, millions head to their doctors' offices for a shot to protect them from the strains of influenza that keep many in bed sick come the winter months.
Simply developing that vaccine is a complicated process, involving guesswork, observation and even a trip to the farm.
The process actually begins in February, as scientists from the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) select the strains of flu virus they expect to be most prevalent in the upcoming year.
Typically they are chosen based on the most prevalent strains in the southern hemisphere during their flu season in the months prior, according to CDC spokesperson Arleen Porcell.
But as past years have shown, selecting the right strains is not a perfect science.
"There's a certain amount of science and a certain amount of art to it," said Stephen Morse, an epidemiologist at the Mailman School of Public Health at Columbia University.
According to Dr. Andrew Pavia, chair of public health committee of the Infectious Diseases Society of America and a professor of professor of pediatrics at the University of Utah, the vaccine has been a good match 18 of the past 21 years.
"Some years the predictions are not perfect," he said.
Because of the WHO involvement in the annual meeting, vaccinations are relatively standard everywhere during our flu season.
"Wherever you get it in the Northern Hemisphere, if it's been made properly, it will be identical," he said.
Ultimately, three strains of influenza virus are chosen for the vaccine: a variant of H1N1, a variant of H3N2 (both influenza A viruses) and a strain of the influenza B virus. Influenza A viruses are the strongest, and vary the most from year to year, while B viruses rarely make anyone but the most vulnerable people sick.
Influenza C viruses are not part of the vaccine, largely because they rarely vary and typically physicians think it would be better to just develop immunity from catching it, said Morse.
The influenza A viruses are named for their variants of hemagglutinin, a protein that helps the flu virus cling to cell surfaces, and neuraminidase, an enzyme that helps it separate from cell surfaces to move in the body.
H1N1 was the strain responsible for the influenza pandemic of 1918-19. It was dormant for many years before it resurfaced in the late 1970s. While the reasons remain unclear, many in the scientific community suspect that it was brought back from a strain someone had preserved in a lab.
H3N2 was responsible for the pandemic of 1968-69. Since no major pandemic has occurred in the past 40 years, H3N2 has remained the dominant strain of the virus.
"When you say you've got the flu... it's usually because of H3N2 circulating," said Morse.
The pandemic of 1956-57 was caused by the H2N2 strain. Since the H component of a flu virus changes more rapidly, the flu vaccine targets that particular protein.
But while the virus remains similar from year to year, some small variations in its structure enable it to slip by the defenses of whatever version the body might have encountered the year before.
"Over time, essentially what happens is that variants, mutants, appear that have an amino acid that has changed a little bit," said Morse.
For this reason, a new flu vaccine is required each season.