The first suspected case of monkeypox in California, a viral infection that causes respiratory symptoms and scarring skin lesions, is under investigation in Sacramento County, the California Department of Public Health said in an announcement Tuesday.

The case, which the Sacramento County Public Health Department began investigating on May 21, involved a resident who initially tested positive for orthopox virus, of which monkeypox is one type. Confirmation of monkeypox is pending testing at the Centers for Disease Control and Prevention, and the patient is in isolation, state health administrators said. Public health staff are conducting contact tracing and after-exposure prevention for close contacts of the person. The patient previously traveled abroad, according to the state.

Currently, there are nine confirmed monkeypox cases in seven states the U.S., according to the CDC. The first U.S. case occurred on May 18 in Massachusetts in a patient who had traveled from Canada, according to the CDC.

Monkeypox causes respiratory symptoms and an outbreak of lesions that can cover large areas of the skin and mucous membranes. Symptoms can include fever, muscle aches, swollen lymph nodes and a rash or lesions with a hard papule. The incubation period is usually seven to 14 days but can range from five to 21 days. Patients are infectious from the onset of symptoms until the skin lesions have healed, according to the CDC.

The lesions can cause scarring. Typical symptoms might not always be present initially, but it can cause three days of shakiness and maybe a little bit of fever, followed by the eruption of the skin lesions, which typically in the past would occur on the face and then spread, said Dr. Stan Deresinski, a clinical professor of medicine/infectious diseases at Stanford. In current cases, the lesions can appear in all sorts of unusual places, which is an indication that it spreads by direct physical contact, he added.

Deresinski said the disease is unlikely to become a pandemic, but he does expect to see more cases, in part because so many people are not vaccinated against smallpox, which would confer immunity.

"There will be more cases, there's no doubt about it. And there will be people spreading it to their friends and neighbors — intimate neighbors — but it's not going to be anything like COVID-19," he said.

But he added a caveat.

"I speak on the assumption that nothing has changed about the virus. It probably hasn't, although we don't have the full genome sequencing based on earlier viruses to which it is related," he said.

Monkeypox was first discovered 1958 in a monkey in a laboratory in Denmark, according to the World Health Organization. The first human case was found in a child in the Democratic Republic of the Congo in 1970. Current evidence suggests that monkeys are not the virus' primary reservoir in nature. It likely originates in small mammals, including many rodents, Deresinski said.

"There was an outbreak of monkeypox in the U.S. in 2003. It was the result of importation of Gambian giant rats, perhaps some other rodents that infected prairie dogs, perhaps in pet stores that were then sold as pets. There were 47 cases of monkeypox throughout the United States. There were no deaths and there was no confirmed person-to-person transmission in this case," he said.

Endemic to western central Africa, the virus is related to smallpox, which was once a scourge and was virtually wiped out by worldwide vaccination. Smallpox vaccines, which also confer some immunity to monkeypox, have not been given since the 1980s when the virus was considered eradicated.

Monkeypox is a DNA virus, unlike SARS-CoV-2, which is an RNA virus. DNA viruses don't frequently mutate while RNA viruses "mutate like crazy," Deresinski said.

While there have been a few limited outbreaks outside of Africa, the current cases in Europe are thought to be spreading at raves, which are large groups of people who congregate to listen to music and dance, which have occurred in Belgium and the Canary Islands, he said. The Canary Islands gathering attracted 80,000 people.

Many of the cases are among people who self-report as males and have had sexual contact with other males. It was then carried to countries throughout Europe and North America, Deresinski said.

Deresinski noted that last July, a traveler who came from Nigeria with skin lesions of monkeypox took two airplanes and flew to Dallas, Texas. An emergency room doctor immediately suspected the disease was unusual, diagnosed monkeypox and placed the patient in isolation.

"The CDC got involved in investigating secondary cases. They identified over 200 potential contacts, so this meant people near him on the airplane, etc., might have been exposed. They identified zero secondary cases. So while it is true that the virus is present in your upper respiratory tract and in some cases may be transmitted after prolonged face-to-face contact, in the current circumstance especially, it seems that most of the transmission is related to direct physical contact," he said.

While monkeypox can also be spread from contact with contaminated objects such as clothing and furniture, it is rare.

"It's clearly not a major issue or we would have seen many more cases," he said.

Deresinski said people shouldn't worry about getting monkeypox, but they should still use common sense. The virus can still confer serious illness and death.

"The mortality rate in Africa, depending on which strain of virus you have and other factors, seems to be anywhere between 1% and 10%. It is anticipated that the mortality associated with cases outside of Africa would be significantly lower. And that's especially true now that we have what may be effective treatment," he said.

The higher rates of mortality in Africa are likely due to other comorbidities such as malnutrition, he said. In developed countries, monkeypox would be most critical for people who are immunosuppressed, he said.

Elderly people in the U.S. who had the smallpox vaccine, a likely target group for serious illness, might actually be better off. They could still have some residual immunity because they were vaccinated with the live virus, which would probably stay with people to some extent, he said. The duration of immunity, however, after 50 years, isn't clear, he added.

There are two vaccines that could prevent monkeypox. One is a single-shot, live smallpox virus. The nation has stockpiled 100 million doses in the event of bioterrorism. The second is a two-shot live virus that has been deactivated so it won't replicate but will create antibodies.

The U.S. Biomedical Advanced Research and Development Authority, which is part of the Office of the Assistant Secretary for Preparedness and Response at the U.S. Department of Health and Human Services, has ordered $119 million of a freeze-dried version of the second smallpox vaccine, known by the trade name Jynneos, which would allow for its first doses to be manufactured in 2023 and 2024, according to Danish biotech company Bavarian Nordic.

Currently, the CDC is not distributing the vaccines but is considering its strategy.

More information about monkeypox can be found on the CDC's website. For information on symptoms, what people should do and what the CDC is doing to monitor and control any potential outbreak, visit cdc.gov/poxvirus/monkeypox/outbreak/current.html.