"If there's one thing I've learned in 30 years of working with this, it's that it's a fickle, fickle virus," said Greg Poland, director of the Mayo Clinic's Vaccine Research Group in Rochester, Minn. "It's the unpredictability of this illness that requires worldwide monitoring."
When the CDC is up and running, Schaffner said, it's like the conductor of a finely coordinated influenza orchestra. Monitoring of the number and severity of illnesses begins at the local and state level, but all of that data gets sent to the CDC, which looks for alarming patterns or signs of emerging pandemics.
That happened in 2009, when an outbreak of H1N1 swine flu prompted the CDC to develop and promote an additional vaccine. Last year, the agency noticed severe pockets of influenza in specific parts of the country, which helped it target public health messages and resources.
"Anything can happen," Schaffner said. "That's why you need that constant surveillance."
And it's not just the flu that has the potential to cause major problems during the current shutdown. The CDC plays an essential role in monitoring infectious diseases like measles and polio, as well as foodborne pathogens, like an ongoing salmonella outbreak that has sickened hundreds of people around the country. Careful surveillance is the only way to link cases in different states or countries as the same emerging problem.
The federal government is also responsible for conducting routine inspections of high-security labs that investigate extremely dangerous pathogens like ebola viruses.
Without a full staff at work, the potential for health crises to develop will only escalate.
The CDC has "furloughed two-thirds of their employees and only have 4,000 people working -- that's a skeleton crew," Poland said. "It does put the nation at some risk."