A man in San Diego was infected with the vaccinia virus — the virus in the smallpox vaccine — in June, 2012 after having sexual contact with someone who'd recently been vaccinated, according to a new report from the Centers for Disease Control and Prevention.

What's more, the infected man also passed the virus along to another unvaccinated individual during sexual activity, a phenomenon known as tertiary transmission, the report said.

The smallpox vaccine contains the live vaccinia virus, which is similar to smallpox but doesn't actually cause the disease. In 1972, the United States stopped vaccinating members of the public against smallpox because the disease had been eradicated. However, in 2002, the Department of Defense resumed smallpox vaccination for its military personnel and civilian employees.

Because the vaccine contains a live virus, it can cause symptoms in vaccinated people, and the virus can spread to others. People can become infected with the vaccinia virus if they touch the vaccination site on a person who has been vaccinated, or if they come into contact with clothing that has been contaminated with the virus. Symptoms of a vaccinia virus infection include a rash, a fever, and head and body aches, according to the CDC.

Since 2002, there have been 115 reports of vaccinia virus transmission between vaccinated and unvaccinated people, according to a review published in 2011. Most transmissions occur through intimate contact, such as sexual relations, or contact between a mother and child. There have also been reports of transmission at gyms.

The new case is the first to report tertiary transmission through sex, the researchers said.

The first man became infected when he had sex with a recently vaccinated man who did not keep his vaccination site properly covered. The man who became infected developed a rash in his anal area and on his lip, and experienced fever, fatigue and nausea. When the infected man later had sex with a different man, the third individual also caught the disease, and developed a rash on his penis and forearm.

Both infected men were treated with vaccinia immune globulin (antibodies, derived from people already vaccinated against the disease, that fight the infection), and both recovered within a few weeks. Because the unvaccinated men were infected with the vaccinia virus, they are now immune to smallpox, said study researcher Rachael Joseph of the CDC's Epidemic Intelligence Service.

The study highlights the potential for the vaccinia virus to spread beyond the immediate contacts of a person who has been vaccinated, the researchers said. It also underscores the importance of covering the vaccination site, as vaccinated people are instructed to do.

The report is published this week in the CDC's Morbidity and Mortality Weekly Report.