It’s 2016. Why is the common cold still so hard to avoid?

Winter is setting in, marking the unofficial height of the dreaded “cold season” in offices and schools across the country.

The common cold is a familiar foe: Maybe you’re fighting one off right now, or stocking up on vitamin C, tissues and canned chicken soup for the long battle ahead. But despite the regularity of colds in daily life, there’s a lot you may not know about them. Where do they come from? Is there actually a link between colds and cold weather? And why don’t we have the ultimate weapon — a vaccine — to beat them once and for all?

As it turns out, far from being just a simple, recurring annoyance, the common cold is more like a Medusa-headed enemy. “There [are] at least six families of viruses that cause common colds in people,” says Fred Adler, a professor of biology and mathematics at the University of Utah.

Not only do these virus families cause similar cold-like symptoms, but they’re really well-adapted to humans. Adler says that rhinovirus, the most common cold virus, likely evolved from enteroviruses commonly found in our gut. Now, at least 150 types of rhinoviruses are known to plague our noses and throats. And unlike influenza, which our bodies can develop lifetime immunity against after exposure, our immunity to rhinovirus tends to fade over time.

In short? “You have a long life to look forward to getting fresh, new rhinoviruses,” Adler says.

As for the old adage that colds come when it’s cold out, Adler points out that cold bugs like rhinovirus actually strike all year long. “We just think of them differently in the summer, and they're maybe more annoying because you're not supposed to get colds in the summer.”

That said, recent research does link cooler temperatures with weakened immune response.

“There's some … recent really exciting data showing that the tissues in our bodies and our noses that are not quite as warm as the rest of our body are more susceptible to getting infected,” Adler says, adding that the takeaway is about what happens on a cellular level. “Our cells that have to spend a lot of their time fighting off viruses are less capable of doing that when they're slightly chilled.”

What’s more, when temperatures fall, our social habits can make it easier for cold-causing viruses to spread. Adler says that the concentration of kids heading back to school (indoors) after breaks is one of the biggest factors in spreading colds and other infectious diseases. Another factor may be that when we spend more time indoors, we get less vitamin D from the sun. “Vitamin D is important in keeping our immune system active,” he adds.

But while we may catch colds from our family members or colleagues this winter, experts say we can cuddle up to pets without worrying about catching their colds.

“Many of these viruses that we associate with colds are really species-specific,” says Jeff Bender, a professor of public health at the University of Minnesota’s veterinary school. He cites a canine influenza outbreak that made landfall in the United States last year. So far, no human infections have been reported, although Bender says it’s best to keep an eye on animal viruses as a matter of course.

“We know that sometimes they're sloppy in their replication,” Bender says, “and there are potential opportunities for interchange of those viruses between animals and people.”

The global outbreak of SARS (severe acute respiratory syndrome) over a decade ago came from a coronavirus with an animal host, and the more recent Middle East Respiratory Syndrome (MERS) did, too. But by and large, Bender isn’t concerned about your dog giving you rhinovirus or other common cold viruses. “These really are human-specific,” he says.

And in the future, we may not even catch colds from our fellow humans. Results from a vaccine trial recently showed that rhesus monkeys injected with a mix of 50 inactive human rhinoviruses could develop antibodies against the viruses. Although the vaccine in its current form can’t protect against all cold-causing viruses, the results are promising.

“It is possible that somebody could load up a vaccine with a lot of these different common colds and induce pretty effective immunity,” Adler says. That said, he suspects that if a cold vaccine does eventually reach clinics, there’ll be the question of whether we use it. “It may not seem worth it to people, given that colds aren’t that severe for most individuals.”

In the meantime, there’s not much we can do to completely ward off colds, save for total isolation — and there are a few anecdotes about that. Like the Shackleton expedition to the Antarctic from 1914-1916, wherein crew members endured pack ice, bitter cold and hunger. 

“Those guys did not get colds," Adler says. “They were so isolated that their population wasn't large enough to support colds at all.”

Duly noted — and maybe that’s just another reason to spend this winter daydreaming about deserted tropical beaches.

This article is based on an interview that aired on PRI's Science Friday.