Tucked away in the northeast, Vermont almost looked like a holdout from the flu virus’ geographic spread last month, according to the CDC’s flu activity map for 2017-18.

But, alas, by the end of December, the region has gone from white to yellow, and then to orange, denoting no activity, local activity and, finally, regional activity, with two or more regions in the state reporting flu outbreaks and laboratory-confirmed influenza.

Dr. Aaron Burley of Essex Pediatrics said he typically sees the virus peak in January and into February before tapering off in March and April.

In 2016-17, the flu was characterized as “widespread” in Vermont from January 29 to April 8, according the Vermont Department of Health’s Influenza Surveillance Report. Emergency room visits due to influenza-like illnesses peaked from March 5-11, according the report.

By late December, Burley had already seen a number of “non-specific” viral illnesses, as well a “surge” in stomach viruses causing vomiting, diarrhea or a combination of the two. He had not yet seen a significant number of flu cases.

Frequently, he said, if symptoms are mild, doctors won’t test for the flu, or it will go undiagnosed altogether.

“When someone has persistent fever with a sore throat, chills, muscle aches and are feeling unwell, and there’s no other explanation, that’s often when people test for it,” Burley said. “And if you’re around people with a compromised immune systems, it’s often tested for.”

The health department distinguishes the flu from a cold if the patient has symptoms such as a high fever that can last up to four days, as well as a potentially severe headache, aches and pains, exhaustion and chest discomfort.

The U.S. sees nearly 9 million cases of the flu each year, 14,000 hospitalizations and 12,000 deaths, with potentially life-threatening complications like pneumonia and bronchitis, the state health department says.

These dangerous complications, as well as the flu’s reach in the U.S. and globally, are what make the flu such a topic of conversation heading into winter, Burley said.

“And the fact that you have to talk about the vaccine each year,” he added. “Because the virus does mutate.”

The influenza virus has three main types – A, B and C – with each having multiple sub-strains. The H and N found in a strain, like H1N1, are different proteins found on the outer shell of the virus, Burley said.

The vaccine’s effectiveness, he said, varies from year to year, depending on how well the virus used to make the vaccine is matched with the actual virus circulating in the community and getting people sick.

The ability to study and predict changes in the virus can be tricky, he said, and if a different strain becomes dominant, people won’t be as protected.

Exactly why the flu season spikes in the fall and winter months is not fully understood, Burley said. But the virus’ life cycle plays a role, plus having school in session and simply being more cooped up in the winter.

“By the time January comes around, we’ve spent a lot of time indoors, close to each other, passing germs around,” he said.

He also said the virus has a predisposition for this time of year. A study performed by the National Institutes of Health and published in “Nature Chemical Biology” in 2008 found the outer membrane of the virus becomes harder in cooler winter temperatures and helps better protect it when moving from person to person.

Testing for the flu is done with a swab, collecting a sample from the back of the throat and nose, but is not usually necessary.

“Most often it will get better with time and rest,” Burley said.

He said the most effective defense again any virus, including the flu, is the simple credo of all healthcare professionals: Wash your hands.