My roommate didn’t get a flu shot last year.

“It costs money to get a flu shot,” she told me. “But it’s free to get sick.” This was perhaps the weirdest logic I have ever heard. Needless to say, she got the flu. She spent the next two weeks stationed on the couch with tea in one hand and tissues in the other.

As her housemates, we steered clear of her at all times except for when we graciously passed along mugs of hot tea and bubbling cups of Airborne. We did these kind things not without mockery, however. We didn’t pass up pointing out the irony in the situation. What she had refused to pay for the flu shot she made up for in tissue use and cough drop consumption.

She isn’t the only one who didn’t immediately head to the nearest Walgreens at the first sign of a sneeze, however. Every year we hear the same public health message to get vaccinated, and every year, there are the people that get the shot, and the people that get the flu, and the people that get angry that reporters won’t shut up about it all.

The thing is, the flu virus is kind of a big deal. Since the early 1900s there have been four flu pandemics, none of which have carried the same characteristics, but all of which have caused fatalities.

The first extreme bout of the influenza virus occurred in 1918 and is commonly called the Spanish flu—you might have heard of it. Approximately a fourth of the world’s population was ill from the virus that year, according to flu.gov, a website that tracks pandemic flu history.

A study from Stanford University documents that the virus was so common, it became a part of children’s rhymes: “I had a little bird, it’s name was Enza. I opened the window, and in-flu-enza,” kids would chant as they played jump rope.

Again in the late 1950s, communities worldwide were struck by a new flu strand identified initially in the east and spreading to the United States by the summer of 1957, killing nearly 70,000 people in the U.S., health records report.

Ten years later another flu virus was discovered. While this pandemic was smaller than the previous outbreaks, the improved medical care could still only do so much. And then, only a few years ago, the H1N1 virus, or swine flu, struck the world as the Center for Disease Control and Prevention (CDC) worked to develop a vaccine.

The U.S. government declared the virus a public health emergency. As people became vaccinated, the impact of the illness began to decline, but the CDC estimates thousands of deaths as a result of the virus.

After each pandemic, after numerous vaccines, after temporarily eradicating a virus, we have not beaten the flu. Viruses are sneaky.

Imagine that you are trying to fight a boggart—a fictional creature from Harry Potter that takes the form of that which you are most afraid. Yes, I am comparing the flu to a mythical character. Bear with me. The boggart is a creature that will never really be the same shape. The form will change depending on who is trying to fight it. It mutates and adapts depending on the fears of an individual.

The flu virus acts in much the same way. Though not likely to take the form of spiders or clowns and other fears, the virus can mutate so that each strand and outbreak are different from the others. So that each new strand requires another vaccine. Doctors and scientists are doing their best to keep up.

My roommate got her flu shot this year. And maybe you should too. Who knows when the next pandemic is on its way.