In the Bay Area, more than two dozen confirmed or suspected cases have already been reported, with 16 in San Francisco alone as of Friday, though that number is updated only once a week and is almost certainly higher by now, health officials said.

The infectious disease, a cousin of smallpox, manifests itself as skin lesions and is spread by intimate, person-to-person contact. Most people recover fully without treatment, but monkeypox can cause severe illness in children and some other vulnerable groups. Even for those who aren’t seriously sick, it can take weeks to get over and cause discomfort and pain.

Almost all cases so far have been reported among gay or bisexual men, most of whom are believed to have been exposed through sexual or other close contact with someone who was infected. The risk to the general public remains very low, local and federal health officials say.

Monkeypox is nothing like COVID, and is not currently — and almost certainly never will be — a threat at the same crisis level, experts say. But the outbreak is at a critical stage where health officials have an opportunity to stamp it out before cases spread further and potentially affect more vulnerable people.

“Monkeypox is not the same scale of a problem (as COVID). That said, if there’s an opportunity to control an emerging disease, it’s important we try to do it,” said Dr. Seth Blumberg, an infectious disease expert at UCSF. “We can’t blow this off. We need the political and societal will to control the disease now.”

It’s possible that monkeypox, if allowed to widely circulate, could become endemic in the United States and threaten the general population, though many health experts said that outcome is unlikely given the nature of the virus and how it spreads, plus the existence of effective vaccines to stop it.

Monkeypox also could establish itself as a recurring threat that triggers fresh outbreaks every few years, especially if it becomes embedded in U.S. animal populations. Or it could join the ranks of sexually transmitted infections, including syphilis and gonorrhea, that affect certain communities and have proved stubborn to control.

“It will suck if monkeypox joins the list of STIs that people have to worry about,” said San Francisco Supervisor Rafael Mandelman, who has called for a hearing this month to discuss the city’s public health response to monkeypox. “We need to be moving quickly on vaccination and stopping the spread now.”

In the Bay Area, pressure is mounting on health officials to make vaccines — which for monkeypox, can work before and after exposure to the virus — more broadly available, and to conduct more widespread surveillance to quickly determine whether the disease spreads beyond the communities currently affected.

Most testing is being done by state laboratories, which confirm results with the CDC. Stanford began providing laboratory testing for monkeypox two weeks ago in anticipation that the outbreak and demand for surveillance could grow quickly, said Dr. Benjamin Pinsky, head of the Stanford Clinical Viral Laboratory.

Vaccination efforts around the country are currently held up by lack of supply. Two vaccines are authorized for prevention of monkeypox, though the preferred product — called Jynneos — is in much shorter supply. The second vaccine, called ACAM2000, has side effects that for some people could be worse than the illness itself.

The U.S. last week announced plans to rapidly ramp up its vaccination efforts with Jynneos. So far, about 66,000 doses have been delivered to states with monkeypox cases; an additional 240,000 doses are expected to go out in the coming weeks, and at least 1.6 million total doses should be available by the end of the year, according to the CDC. On Friday, the U.S. Health and Human Services Agency said it had ordered an additional 2.5 million doses, which will be available at the end of this year and in 2023.

California, which has about a quarter of all cases in the U.S., expected to receive about 15,000 doses by the end of last week or early this week. Jynneos is administered in two doses given 28 days apart.

Bay Area counties reported receiving anywhere from 10 doses to more than 500, in San Francisco. That’s not enough to offer vaccination to everyone who might want it, so local health officials say they are targeting only those with a known exposure at the moment. That includes people identified through contact tracing of reported cases, people who hear informally that a partner was recently diagnosed, or those who attended an event or venue associated with one or more monkeypox cases.

“We don’t have enough for everybody,” said Frank Strona, the incident management lead for the San Francisco Department of Public Health’s monkeypox response. Strona said more than 200 doses had been administered in the city as of Friday morning. “We anticipate more batches every few days,” he said.

Once more vaccine becomes available, officials said, they hope to offer it to people at risk of becoming infected but who don’t necessarily have a known exposure. The vaccine may not ever be needed for the general public if the outbreak is contained.

Monkeypox tends to cause flu-like symptoms and a trademark rash, with dense, fluid-filled lesions. Most people are sick for two to four weeks and don’t need treatment, though a few drugs are available for severe cases. Worldwide, a handful of deaths have been reported this year, but none in the U.S.

This year’s global outbreak, which has so far infected more than 5,000 people, has baffled infectious disease experts who have never seen the virus spread much beyond the West African countries where it’s endemic. The United States would typically see a few travel-associated cases every few years, but previously had reported only one outbreak: In 2003, 47 cases were identified, all linked to rodents imported from Ghana.

It’s not clear why monkeypox suddenly took off, though experts suspect it may be a combination of the virus mutating to become more transmissible and finding traction in groups where it could spread quickly and easily.

Monkeypox transmits primarily through direct, sustained contact. People are most at risk if they are exposed to the fluid inside lesions, for example by touching the rash of an infected person or sharing bedding or towels with someone who’s infected. The virus can also pass from person to person through respiratory droplets, but only at close range — it doesn’t spread through the air of restaurants and grocery stores.

In the U.S., 271 of the first 305 cases were in men, and more than 70% were men who have sex with men, according to the CDC. Several large clusters have been traced to events or venues — including private sex parties and clubs or bathhouses — where people had sex with multiple partners.

“A small number of people have a large number of sexual contacts, and that can cause very rapid and early spread,” said Dr. Jason Andrews, an infectious disease expert at Stanford. “But it doesn’t necessarily mean it will be sustained that way.”

Health officials note that unlike with HIV and some other sexually transmitted infections, there’s no connection between particular sexual practices and the spread of monkeypox — the virus just happens to have taken root first in gay networks. It can spread as easily among heterosexual partners, or among close household contacts.

Andrews said he suspects the global outbreak may be starting to slow after weeks of explosive growth. He and other infectious disease experts noted that because of a long incubation period for monkeypox, cases being diagnosed now are probably from exposure one or two weeks earlier.

The possibility remains for the virus to get a foothold in the broader population. “The most concerning outcome would be if it spreads more broadly as an endemic infection across all ages, or through casual contact routes,” Andrews said. “I don’t think we have strong evidence of that happening right now, but we have to prepare for it.”

Erin Allday is a San Francisco Chronicle staff writer. Email: eallday@sfchronicle.com Twitter: @erinallday