Type in "gatherings" and it appears 34 times next to family, social, mixed households.
In the Bay Area alone, it's the reason cited for five of the nine counties as a driving force in rising numbers.
"First of all it's the risks we would think about -- lots of noses and mouths together close by indoors versus outdoors," said infectious disease expert Dr. Peter Chin-Hong with UCSF, explaining the dangers of private, social gatherings.
WATCH LIST: 38 California counties where COVID-19 is getting worse
San Mateo County's public health officer issued a stern letter about social gatherings as a main factor for an increase in new COVID-19 cases, just one week before it was put on the list Wednesday.
In Solano County in particular, 95 percent of recent cases happened that way and 5 percent were work clusters, none from community transmission.
"The lesson is very clear. If people practiced social distancing in their personal lives, this disease, this outbreak diminishes and the surge goes down," said Dr. Bela Matyas, public health officer for Solano County.
Here's the issue -- go to any public space and the restrictions are plain to see -- signs that tell you to stay six feet apart and wear a mask and some businesses are shut down all together.
But there's no way to regulate what happens in homes, backyards and private spaces.
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Social Bubbles, gatherings of 12 and fewer, are technically allowed in San Mateo, Contra Costa, Alameda, Marin, San Francisco and Santa Clara counties.
The bubbles aren't allowed in Napa, Sonoma and Solano. But with public health officials still grappling with enforcing masks and social distancing, we're not hearing anyone getting a ticket for gathering in their own private spaces.
In San Francisco, public officials decided not to take any action against a Catholic church after hosting a wedding in July where at least 10 people tested positive for the virus.
RELATED: Getting off state's COVID-19 watch list nearly impossible, Solano County public health official says
"Since all we can regulate is the public environment those regulatory changes aren't having any effect on the outbreak, so nothing we've done has been making a difference," said Dr. Matyas about enforced shut downs or other rules in public areas.
Dr. Chin Hong says the real problem with social gatherings is that people are very quick to let their guard down.
"It's the familiarity aspect, it's the emotional aspect," he said. "You don't think of your Uncle Harry or your Aunt Sue as being purveyors of COVID-19. It seems kind of rude to wear your masks and hug your Uncle Harry."
That's why his advice goes beyond just staying in a small social bubble. He suggests only hanging out with people who share the same COVID-19 values. That means there's an understanding within the group about what rules to follow and how strict the enforcement.
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