Thousands in California and Massachusetts are being monitored for coronavirus in preparation for an outbreak in the U.S., officials from each state have reported.

What are the details?

More than 8,400 people in California are being monitored for COVID-19, or commonly known as coronavirus, Gov. Gavin Newsom announced on Thursday, also confirming that 33 people in the state have contracted the virus.

On Wednesday, the state reported the first coronavirus contraction without a known cause in the U.S., leading to speculation that a "community spread" may be starting. The patient had not recently traveled to a foreign country and had not been exposed to another known infected person, authorities said.

"We are not overreacting but we are not underreacting," Newsom said in the news conference. "Yesterday's case generated a lot of attention, but we all knew this was inevitable."

The news follows an announcement from Massachusetts Public Health Commissioner Monica Bharel that over 600 in that state have been quarantined while being monitored following their return from China.

The Massachusetts individuals — 608 in total — have voluntarily placed themselves under a 14-day quarantine in their homes, according to Bharel. Of the quarantined individuals, 377 have completed the monitoring with no symptoms, but 231 are still completing the process and are being monitored.

Bharel stressed that the health risk for the area "remains low," and that there is currently no "community-level spread" in the state.

"We're preparing for whatever comes our way here in Massachusetts," she added. "People should live their lives normally, and go about their normal activities."

Health officials say an epidemic in the US is inevitable

On Tuesday, the Centers for Disease Control and Prevention warned Americans to prepare for the spread of the disease in communities across the country, saying it is not matter of if — but when.

Nancy Messonnier, director of the Centers for Disease Control and Prevention National Center for Immunization and Respiratory Diseases, said that "as more and more countries experience community spread, successful containment at our borders becomes harder and harder."

Messonnier also said that two out of three "requirements" for a "pandemic of new disease" have been met, and thus Americans should start to prepare for a mass spread.

"It's not a question of if this will happen, but when this will happen, and how many people in this country will have severe illnesses," she added, warning that "disruption to everyday life might be severe."

To date, the disease has infected over 80,000 around the world and has resulted in over 2,700 deaths. While most of the infections and fatalities have occurred in China, experts fear that the virus will begin to spread rapidly to the rest of the world.