(CNN) For months, health experts have warned that all it takes is one infected person to create an outbreak.
Evidence of such coronavirus spreads has already been seen in many parts of the country this year -- including at funerals, weddings and even small to medium-sized get togethers with friends.
The US Centers for Disease Control and Prevention has said that superspreading events, as they are often called, "are associated with both explosive growth early in an outbreak and sustained transmission in later stages."
Now, as Thanksgiving quickly approaches, health experts are worried holiday gatherings -- even if they are not large in size -- could result in massive spreading nationwide.
As of Thursday, the US has reported over 11 million coronavirus cases and 250,000 deaths. Coronavirus can commonly spread "through respiratory droplets or small particles, such as those in aerosols," which are produced even when a person breathes, according to the CDC.
"One of our concerns is people over the holiday season will get together and they may actually be bringing infection with them to that small gathering and not even know it," Dr. Henry Walke, Covid-19 incident manager for the CDC, told reporters in a conference call on Thursday.
Even intimate gatherings can have consequences.
In November, an Election Night watch party, attended by at least six people, created a small outbreak in Fresno, California.
Serop Torossian, Kaiser Permanente government affairs manager, hosted the party at his home with guests that included Fresno Mayor-Elect Jerry Dyer, Fresno County Supervisor Steve Brandau and Darium Assemi, a local businessman.
Brandau admitted he was experiencing symptoms while at the party. He has not responded to CNN's requests for comment.
"It's not lost on me that this outbreak of Covid came through me," Brandau said in a Facebook Live video.
It is unclear who else has been infected, but Brandau said "six or seven" people had tested positive and a public health contract tracer estimated he exposed about 45 people to the virus, including some in his county office.
"The worst part of my decision-making process is I did still go to the dinner party," Brandau said on Facebook. "I did not have an abundance of caution."
This summer, a wedding reception in Millinocket, Maine, led to similar outbreaks in the state.
The wedding, which took place in August, had at least 55 people in attendance. Guests were seated close together, were not made to wear masks, and did not socially distance. Thirty people who attended the event later tested positive for Covid-19.
That reception led to three separate Covid-19 outbreaks, leaving 178 people infected, three hospitalized and seven dead, state health investigators said. None of those who got seriously ill or died even went to the wedding, and many lived far away from the event.
"This report provides a cautionary tale for people as they consider how to celebrate the winter holidays," said Robert Long, communications director for Maine's Center for Disease Control and Prevention. "The gatherings at the center of this outbreak occurred in a rural area that had seen almost no evidence of Covid-19."
And even early on in the pandemic, two funerals in Albany, Georgia, in February and March, helped turn the small southern city into one of the nations' worst coronavirus hotspots.
"It was apocalyptic," said Chris Cohilas, chairman of the Dougherty County Board of Commissioners, where Albany is located. "It gripped us and got into a subset of some of our most vulnerable folks."
More than 100 people gathered for the funeral of Andrew J. Mitchell in February in Albany. Days later more gathered for the funeral of another man.
The two funerals sparked a rapid outbreak that has left many officials and residents still reeling. Dougherty County, which includes Albany and has a population of 85,000, became a close second in confirmed coronavirus cases to Fulton County, which includes Atlanta and has a population of about 1 million people.
"So many people got sick," Alice Bell, whose mother lived with Mitchell, told CNN. "It was truly a domino effect, it was bad."
Cohilas said when coronavirus hit, the city had to take drastic measures.
"We literally had to order a mobile morgue because we had such a high number of fatalities in order to support those who are dying," he told CNN.
"I watched as people that I knew who were connected to folks that had been exposed, some lived and some didn't. It was like watching a snowball roll down and hill and get bigger and there was no way you could stop it. It took a long time to slow down that snowball."
The spike eventually slowed down around May. The county also became the most heavily tested outbreak on a per capita basis, Cohilas said.
The commissioner advises people across the US to pay attention to how they behave during gatherings this holiday season. Washing hands, social distancing and wearing masks could save lives, he said.
Canada saw a spike in cases just three weeks after their country celebrated its Thanksgiving holiday on October 12.
Dr. Bonnie Henry, British Columbia's provincial health officer, said she pleaded with people to keep their gatherings small. She also announced a ban on inviting more than six people into a home.
"Unfortunately, there were a number of events that have happened that have led to quite dramatic increases," she said earlier this month.
One Thanksgiving party with an extended family actually led to 10 Covid-19 infections, including three babies, according to York Region Public Health, a health unit north of Toronto.
The virus also spread to another household, infecting four more people, and to a workplace where two more people were infected with the virus.
Cases like that and other helped fuel the spike in cases.
"I think we need to consider all the celebrations that are coming up whether it's Diwali, or Hanukkah, or Christmas," Henry said, "and look at how we can regroup and focus on our immediate families and making sure we can support each other to do it safely."