The Bay Area had some of the country’s first cases of the potentially deadly new coronavirus, and as infections reached pandemic levels around the world this week, more and more people locally are testing positive for the disease — with no signs of abating.

There are more than 200 reported cases in the region. But why are some counties seeing spikes while others are not?

In Santa Clara County, cases soared from a single one on Jan. 31 to more than 110 on Sunday, while Napa County has yet to confirm a case. Alameda County — home to the University of California at Berkeley, an international draw with students and faculty from around the world — has just a handful.

“It’s just really hard to know how much of it is luck and how much of it is under-detection,” said Arthur Reingold, division head of epidemiology and biostatistics at UC Berkeley.

Those tested for the disease known as COVID-19 had either known exposure through foreign travel, contact with infected people or severe symptoms that drove them to seek medical care. But many people who are infected don’t feel sick or only mildly ill, though they can still transmit the virus. As a result, they don’t seek treatment or get tested, potentially masking the true extent of the disease in the Bay Area, Reingold said.

For example, if one county has an infected carrier without symptoms who’s a social butterfly, and another has one who’s a homebody, the extent of transmission in those counties will be quite different, Reingold noted.

“There can be variation just due to chance alone,” he said.

Bottom line, a low infection count in one community and a higher count in another doesn’t necessarily mean one is safer than the other, or offer an excuse to slack off on social distancing and hand-washing anywhere in the Bay Area.

“First and foremost,” Reingold said, “we want people to improve their hygiene and social distancing wherever they are.”

Here’s a look at how coronavirus cases are spreading around the Bay:

Santa Clara County

No county in the state has been as hard hit as Santa Clara. Since Jan. 31, when it became the first in Northern California to confirm a case, reported infections have grown to 114 as of Sunday, March 15. The state was reporting a total of 335 cases as of late Saturday. Santa Clara County is also home to two of California’s six known coronavirus fatalities.

The county’s first case — at the time only the third in California and seventh in the U.S. — involved a local man who had become ill after returning Jan. 24 from traveling to Wuhan City in China, where the outbreak originated in December. On Feb. 20, he became the county’s only case to date declared to have recovered.

Of the total cases, 15 were associated with travel to China or other infection hot zones, 28 had been in close contact with infected people and 52 involved patients with no travel risk or contact with known cases, indicating their infections were acquired through the local community. The county reported its first case of presumed community transmission Feb. 28.

Of Santa Clara County‘s known cases, 48 required hospitalization. The county reported its first COVID-19 fatality on March 9. Two days later, the White House singled out the county along with three others in the Seattle area for special restrictions on large gatherings to quell the spread of the disease. A second death came Friday.

Alameda County

Alameda County reported its first case March 1, involving a health care worker at NorthBay VacaValley Hospital in Solano County who was exposed to an infected resident there. Alameda County’s total to date is 15 as of Sunday, March 15. Of those, two were believed infected through community transmission of the disease. One was a passenger aboard the Grand Princess cruise ship, which sailed from San Francisco to Mexico in February. A Placer County man on the trip has since died from the disease. The rest of the county’s cases had contact with other confirmed cases.

Berkeley Public Health, which tracks its cases separately from the county, has reported three cases, one of which was aboard the Grand Princess trip to Mexico.

Contra Costa County

Contra Costa Health Services on Feb. 25 confirmed that local hospitals were caring for three people who had tested positive for the disease after they were evacuated from the Diamond Princess cruise ship in Japan, which was stricken by an outbreak, to Travis Air Force Base in Solano County.

On March 3, Contra Costa county confirmed its first case, and said that person had no known travel risk or exposure to other infected people and likely acquired the disease in the community.

Cases have multiplied rapidly since. On Sunday, March 15, the count was 29, more than tripling in a week, with no deaths. The county hasn’t specified how many of its cases were associated with known travel or exposure risks, but as of Sunday at least four had been deemed community acquired, and two were passengers on the Grand Princess Mexico cruise. In addition, the county still has a non-resident patient from the Diamond Princess in a local hospital.

San Mateo County

San Mateo County reported its first case Feb. 29 involving a patient who was repatriated to the U.S., and two days later, confirmed its first suspected case of community transmission of the disease. By Sunday, March 15, the county had reported one death and the count had leaped to 31.

County Health Officer Scott Morrow has not said how many of those cases were community acquired or associated with foreign travel or exposure to the infected, citing privacy concerns, but said Tuesday that he has “evidence of widespread community transmission of COVID-19 in San Mateo County.” The county has not reported any deaths from the disease.

San Francisco

San Francisco health officials on Feb. 3 acknowledged that a San Benito County man who had become ill after returning from China and passed the infection to his wife were both being treated at a hospital in the city.

On March 5, San Francisco announced that two unrelated residents — a man in his 90s and a woman in her 40s — had tested positive with no known exposure risks suggesting community acquisition of the disease. Two days later, the city reported six additional cases, all involving exposure to other confirmed cases.

By Sunday, March 15, that figure had more than tripled to 37. The city had no breakdown of how many of those were community acquired, but still has no reported fatalities.

Marin County

As of Sunday, March 15, the county reported 5 cases and no deaths. Marin health officials on Feb. 27 announced that a non-resident evacuee from the Diamond Princess cruise ship who had tested positive after being taken to Travis Air Force Base in Solano County was moved to a hospital in Marin and has since been sent home.

On March 9, the county announced the first infection in a local resident, an older man who had traveled to Mexico aboard the Grand Princess.

The county on Wednesday reported two additional infections involving close contacts of the resident who had been aboard the Grand Princess, and said they had mild symptoms and were being treated at home.

Sonoma County

Sonoma County on Feb. 25 disclosed that a person who had been taken from the stricken Diamond Princess cruise ship in Japan to Travis Air Force Base had been moved to the county for care after testing positive.

On March 2, the county reported that a resident who had traveled on the Grand Princess trip to Mexico was presumed to be infected. On March 5, the county reported that a second resident passenger on the ship also had tested positive, bringing the total to three including the first non-resident case.

County health officials noted that 78 residents were on that cruise, and 25 — including the two who tested positive — had taken a shuttle to Sonoma County Airport on Feb. 21. On March 15, the county reported it had two cases of community spread, bringing the total number of cases to five.

Napa County

The heart of California’s Wine Country has stood alone among Bay Area counties in having no reported cases of the new coronavirus to date.

On Feb. 17, two patients from Travis Air Force Base with symptoms of COVID-19 arrived at Napa’s Queen of the Valley Medical Center. On March 6, Napa County reported that as many as 20 residents had been on the Grand Princess cruise to Mexico.

But so far, testing has not confirmed any infections among residents.

Solano County

On Feb. 27, Solano County reported its first coronavirus case — one that marked an ominous development in the outbreak — involving a woman with no known history of exposure to the disease through travel or contact with an infected person. It marked the first U.S. case of COVID-19 spreading in the community. That news was followed days later by a similar community-acquired case in Santa Clara County.

The Solano County case raised questions about whether the repatriation of U.S. citizens in China and those aboard the Diamond Princess cruise ship in Japan may have helped spread the disease locally. People from both groups were quarantined for 14 days at Travis Air Force Base in Fairfield — the first group arriving there in early February, the second more than two weeks later.

On Feb. 28, Solano County announced that a resident quarantined at Travis from the Diamond Princess had tested positive, and that a second resident who had been on the ship and taken to the base was presumed infected as well. Both cases occurred well after they had been released from the 14-day quarantine.

On Mar. 1, county officials confirmed that a NorthBay VacaValley Hospital health care worker had also tested positive after being exposed to Solano County’s first case, as did a coworker from Alameda County. Solano County now counts six cases.

Santa Cruz County

Santa Cruz County announced its first case March 6 in a passenger who had been aboard the Grand Princess cruise trip to Mexico. Two days later, the county reported a second case in a person who had traveled to Seattle, another national hot zone for the disease. The county confirmed additional cases March 10 and 11, and by Friday the tally had risen to seven, and officials said they considered the virus to be spreading in the community.

San Benito County

San Benito County reported Feb. 2 that a resident who had traveled to China had become infected and also infected his wife upon his return. They were moved to a hospital in San Francisco for treatment after their conditions worsened, but subsequently released and were declared recovered Mar. 4. The county has since reported one other case as of Saturday, March 14.