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In 1968,hot sex videos in trousers scientists discovered a new strain of flu circulating around Hong Kong. The virus, though, didn't stay put. It soon left Asia and turned into a proper pandemic, traveling around the globe and killing one million people worldwide, including 100,000 in the United States that season.

The deadly virus struck in the U.S. when it usually does, during winter. That year, "kids didn’t care about when Santa came," remembers Susan Donelan, who is now a medical director and assistant professor of infectious disease at Stony Brook University’s School of Medicine.

Fifty years later, the Hong Kong flu, known more formally as H3N2, still exists, periodically popping up from year to year.

"Now it's considered one of our seasonal viruses," said Donelan, in an interview.

But the Hong Kong flu is an especially infectious strain of the virus. It has the ability to mutate both during and between flu seasons (more so than other strains), rendering our preventative vaccines less effective.

"By the time we’re ready for the flu season, the strain has kind of changed itself," Neha Nanda, a hospital epidemiologist and medical director of infection prevention at Keck Medicine of the University of Southern California, said in an interview. "That’s why it’s a nasty strain."

And like most years, scientists knew the Hong Kong flu would make an unwelcome appearance during the 2017-2018 flu season.

Doctors predict what flu will hit the U.S., and accordingly, what vaccines to produce, based upon what influenza has been dominant in the Southern Hemisphere -- places like Australia. "Every year we look at the Southern Hemisphere," Shane Speights, dean and associate professor of Medicine at New York Institute of Technology College of Osteopathic Medicine at Arkansas State University, said in an interview. "The Southern Hemisphere dictates what goes into our vaccine."

So, by watching events unfold in Australia in 2017, where the Hong Kong flu was dominant, doctors knew what to expect and prepared as best as they could. But in this strain's typical fashion, it has likely mutated, rendering our vaccines less effective and resulting in the Hong Kong flu's remarkable 2018 spread.

#Flu activity continues to increase nationally in the United States. CDC recommends flu vaccination, even if you've already been sick, and early antiviral treatment for people who are very ill OR those who have flu and are at high risk of complications. https://t.co/KHXucF48vi pic.twitter.com/7oJK4qbUp8

— CDC (@CDCgov) January 26, 2018

"The number of people impacted by it [the flu] has been huge," said Nanda.

The CDC, as of January 20, reports "widespread" flu activity around the country, including the deaths of 37 children.

Even though some of us may have been exposed to H3N2 in previous seasons (for example, the CDC identified six variants of H3N2 during the 2015-2016 season), the exterior of the virus -- home to a variety of specific proteins -- may have mutated too dramatically for us to have any substantive immunity from this earlier exposure.

"The surface proteins change," said Nanda. "Our body may not remember what we experienced two seasons back."

While scientists seasonally do a pretty good job of predicting what will hit the U.S., much less is understood about why strains like the Hong Kong flu wreak havoc for a season or two and then become less dominant or disappear, only to return once again.

"We need a crystal ball," mused Nanda.