Johnny Bocanegra and son Enzo were in their seats early at Chase Center. They were among the 18,000 who filled the arena Thursday night to see Stephen Curry’s return. And, like most on hand, they weren’t going to let coronavirus fears derail their fun.

“I’m just here to have a good time with my son,” said Bocanegra, who got tickets for Enzo’s ninth birthday and drove from Hollister (San Benito County). “I’m not worrying.”

Inside the mass gathering at Chase, the primary evidence of contagion concerns was strategically placed hand sanitizer. But outside, fears of the virus’ spread continued to mount. San Francisco’s first two cases were confirmed that morning, joining dozens reported across the Bay Area.

The coronavirus has impacted every sector of life, including the sports world. Sports is a vehicle for gathering large groups of people, from disparate parts of the community, who come in close contact with one another. What’s the point of going to a game if you can’t high-five total strangers, share some peanuts, scream loudly in a crowded space and maybe even engage in a bro-hug?

The impact of the coronavirus in the sports world is sure to grow.

“Each individual has to weigh their own risk tolerance,” said John Swartzberg, an infectious disease expert and professor at Cal’s School of Public Health. “If things remain as they are, or get worse, I think the prudent thing to do would be to not go to things that are not essential.

“And I consider sporting events not essential.”

The effects are being felt. With more than 102,000 cases and more than 3,400 deaths worldwide, sports competitions around the globe have been canceled, postponed or played in stadiums without fans.

On Friday, San Francisco Mayor London Breed issued a list of “aggressive recommendations,” urging sports events to be canceled and people to stay away from large gatherings for the next two weeks. The Warriors issued a release soon after, saying that Saturday night’s game against Philadelphia would be played and emphasizing the team’s health procedures.

The NBA has asked teams to start developing contingency plans that include playing a game without fans in the stands and only essential staff present, a league source confirmed to The Chronicle on Friday night.

San Francisco’s new guidelines followed those released a day earlier by the Santa Clara County Public Health Department, which recommended that “mass gatherings and large community events where large numbers of people are within arm’s length of one another” be canceled. Both the Sharks and the Earthquakes also announced their weekend games will proceed as scheduled.

More Information Fan precautions The Warriors asked sick fans to stay home and issued the following recommendations for Chase Center guests: Wash hands often and for at least 20 seconds, especially after using the bathroom, before eating, and after blowing your nose, coughing or sneezing. If soap and water are unavailable, use an alcohol-based hand sanitizer with at least 60% alcohol. Avoid close contact with people who are sick. Avoid touching your eyes, nose and mouth. Cover your cough or sneeze with a tissue, then throw the tissue in the trash and clean your hands.

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This week, Stanford announced it is restricting attendance at sporting events, allowing its venues to be only one-third full, a decision that will impact the first and second rounds of the women’s basketball NCAA Tournament to be held at Maples Pavilion this month.

The virus already has interrupted opportunities for high school athletes; less high profile than professional or collegiate sports, but not less meaningful for the students. The Lowell High School’s top-seeded girls soccer team had to forfeit a playoff game against Chico. Lowell was closed this week when it was learned that a relative of a student had tested positive for the virus. The girls’ basketball team at Menlo School-Atherton also fell victim, following the school’s closure when a relative of an employee tested positive. The defending state Division II team first had its playoff game with Bishop O’Dowd postponed a day before it was canceled Friday, ending the season.

Almost everyone acknowledges the situation is very fluid and very uncertain. Warriors President and COO Rick Welts has been in contact with both the city of San Francisco and the league.

“We are keeping the building as pristine as we can,” Welts said Thursday of Chase Center, where the Warriors have nine remaining home games. “We’ve added staff. We are disinfecting every seat in the building after every game.

“We hope everyone who is at risk is smart in their behavior. This is moving so fast, I don’t think anyone can predict where it will go.”

More large sports gatherings, in the form of Major League Baseball games and NCAA Tournament basketball competition, will begin this month.

To keep athletes, fans and staff safe, organizations have been issuing the same guidelines that the rest of the population has been given: wash your hands, stay home if you are ill.

The Warriors’ team doctor met with players and staff this week to emphasize precautions. Among them, the NBA has recommended that players fist-bump instead of high-five their fans.

Both the A’s and Giants have held team meetings with their medical staffs. The A’s will have their players autograph baseballs and toss them into the stands in lieu of autograph sessions. The Giants will pre-sign items ahead of this weekend’s annual investor barbecue.

“We are monitoring it on a daily basis,” said Giants President and CEO Larry Baer. “We’re in constant conversation.”

Baer said the Giants’ spring training attendance has been “steady” this year, with no noticeable dips. He said the team has no contingency plans about what might have to happen with regular-season games at Oracle Park, not quite a month away (the Giants’ home opener is April 3).

Some athletes are making their own decisions about public contact. Raheem Mostert, the 49ers’ running back, announced on Instagram he would skip this weekend’s autograph show at the Santa Clara Convention Center because of coronavirus concerns. He said his decision was made to prioritize the health of his family: wife Devon, infant son Gunnar, and an unborn child on the way.

Daniel Rascher, a sports-economics professor and director of USF’s Sports Management Program, predicts attendance at sporting events will begin to be affected, especially in hot spots like Washington state.

“If it gets worse, I could see attendance issues with baseball in Seattle, and with March Madness games, because it requires travel to get to those games,” Rascher said.

One of the USF program’s study-abroad courses, which was supposed to visit Olympic venues in Tokyo, has been converted to an online course.

Technology might provide some relief. Rascher thinks sports teams and organizers can use the crisis as an opportunity to promote their streaming services and the use of virtual reality, to bridge the gap between the in-game experience and health concerns.

“This could be a chance for the VR industry,” Rascher said. “Instead of just saying, ‘Hey we can’t play in front of fans,’ take advantage of it. Discount the VR app. Make favorite ballpark foods available through Door Dash.”

For now, as Cal’s Swartzberg said, people must decide how much risk they will tolerate. Though he deems sports nonessential, many of the thousands of happy fans at Chase for Curry’s return might argue otherwise.

“I’m looking at this through the lens of a physician and public health professor,” Swartzberg said. “If things continue as they have been, I would encourage people not to go (to large events). It’s very hard for me to condone the idea of doing anything that throws more gasoline on the fire.”

Ann Killion is a San Francisco Chronicle columnist. Email: akillion@sfchronicle.com Twitter: @annkillion