It's the largest U.S. sporting event to be called off over concerns about the spread of the disease. The South by Southwest Festival in Austin, Texas, was canceled Friday.

The announcement came Sunday night after many players were already in the desert practicing. Qualifying matches were to begin Monday with women's main draw matches starting Wednesday and the men's draw beginning Thursday.

"We're here and still deciding what's next," tweeted Rafael Nadal, the world's second-ranked player. "So sad for all that is happening around the world with this situation. Hopefully soon solutions from the authorities. Stay all well and safe."

The Riverside County Public Health Department declared a public health emergency for the desert cities 110 miles east of Los Angeles, including Indian Wells where the ATP and WTA tours were to play the two-week tournament.

Gov. Gavin Newsom has declared a state of emergency. California has reported 114 cases of the virus.

"There is too great a risk, at this time, to the public health of the Riverside County area in holding a large gathering of this size," Dr. David Agus, professor of medicine and biomedical engineering at the University of Southern California, said Sunday. "It is not in the public interest of fans, players and neighboring areas for this tournament to proceed. We all have to join together to protect the community from the coronavirus outbreak."

The event typically draws upwards of 450,000 fans. It is commonly referred to as the "fifth slam" because of its popularity among the players and its stature, ranking points and over $17 million in prize money that place it one rung below tennis' four Grand Slam tournaments. This year's field includel Nadal, Novak Djokovic, Serena Williams, Naomi Osaka and Coco Gauff. Kim Clijsters was set to continue her comeback at the event.

"So sad to hear the news about the postponing of @BNPPARIASOPEN," Gauff tweeted. "I was so excited to make my debut in IW, but safety is always the no. 1 priority. Stay safe."

Tournament director Tommy Haas said organizers are prepared to play the event on different dates and will explore options. However, the pro tennis calendar is tightly scheduled and the summer months in the desert are notoriously hot.

"We are very disappointed that the tournament will not take place, but the health and safety of the local community, fans, players, volunteers, sponsors, employees, vendors, and everyone involved with the event is of paramount importance," Haas said in a statement.

Already some smaller tennis events in China and Italy — the two countries hardest hit by the virus — had been affected. The Miami Open, which follows Indian Wells later this month, could be jeopardy. The Ultra music festival in that city has already been canceled. The year's first Grand Slam tournament, the French Open, is set to be played in Paris in May.

The decision to postpone Indian Wells was based on the guidance of medical professionals, the Centers for Disease Control and state of California officials, tournament officials said.

"We understand the decision which has been made in the interest of public health and safety which is the top priority at this time," WTA chairman and CEO Steve Simon said in a statement. "It is too soon to speculate about what will happen to other tournaments that follow. We will continue to closely monitor the situation. Health and safety will always come first."

Refunds for this year's event or a credit toward next year's tournament are being offered.

Riverside County health officials said the individual with the first case of locally acquired coronavirus is being treated at Eisenhower Health in nearby Rancho Mirage after testing positive. The person is not being identified because of confidentiality rules.

Heath officials are following up on people who may have been exposed and an investigation is underway to find out how the person contracted the disease.

It's the second case recorded in Riverside County. A cruise ship passenger from Riverside County was diagnosed with COVID-19 recently and is recovering at a Northern California medical facility. That person hasn't returned home since leaving the Diamond Princess ship.

"We have always known this was a possibility," Dr. Cameron Kaiser, Riverside County public health officer, said of the first case. "We have been planning for weeks and are prepared to take the necessary steps to protect the health of our local community."

A charity event featuring Nadal set for Tuesday night at Indian Wells Tennis Garden has been canceled, tournament spokesman Matt Van Tuinen said.

Nadal was set to be joined by defending BNP Paribas Open champion Dominic Thiem, Daniil Medvedev, Matteo Berrettini, Stan Wawrinka, Milos Raonic and American Taylor Fritz for the Eisenhower Cup, a $150,000 winner-take-all event.

Earlier in the week, officials had announced several measures to protect players, fans and staff at the tennis tournament.

Ball kids were going to have to wear gloves and not touch the towels of players on the court. Organized player and fan interaction was also going to be limited at the tournament. Besides ball kids, restaurant and food supply workers were going to wear gloves as well as volunteers taking tickets at entrances. Over 250 hand sanitizing stations were set up throughout the venue.

Other major sports and entertainment events in the desert are scheduled for next month.

The LPGA Tour is to play the first women's golf major of the year, the ANA Inspiration, on April 2-5 in Rancho Mirage.

The Coachella Valley Music and Arts Festival in nearby Indio is set for April 10-12 and April 17-19. It typically draws 250,000 people over two weekends. The Stagecoach Festival, featuring country music, is scheduled for April 24-26 at the same Indio venue.